There is also evidence of the existence of minute marginal plates on each side of the groove.

7.

The hydrospires are ten elongated sacks, each with two deep folds. They are perfectly homologous with those of Pentremites, only differing therefrom in not being united in pairs; consequently there are ten spiracles instead of five. The mouth, or oro-anal orifice, is larger in proportion to the size of the body than it is in Penthrough a specimen which has tremites. Mr. Meek informs me that the all the hydrospires preserved. mouth in some of the Blastoidea is prospires; p, pore leading into the tected by a single valve that covered it like the lid of a jug. From the structure of the orifice, I am inclined to think that in Nucleocrinus it possessed a similar protection.

Fig. 7. Transverse section

h, the two anterior hydro

hydrospire; g, one of the grooves.

In the apex, nearly all the space within the circle of apertures is covered by a thin integument of small plates, fig. 3.* When this is not preserved, a large sub-pentagonal aperture is seen, as shown in fig. 5. This aperture occupies the position of the mouth in the existing echinoderms. The integument, as will be shown further on, represents that which covers the mouth of an embryonic Star-fish. Mr. Conrad described this genus in 1842, as having only one aperture in the summit. genus differs from PENTREMITES, Say, in having only one perforation at top, which is central." (Jour. Acad. Nat. Sci. Phil., vol. viii, p. 280, pl. xv, fig. 17). His figure represents the fossil with the apex downward. Dr. Ferd. Roemer, showed that, when perfect, there is no central opening, and he made this one of the grounds for separating the genus from Pentremites. He described the apex as being provided with six apertures, five of which were divided by a partition within each. These he considered to be the ovarian orifices. The sixth he supposes to be both mouth and vent, which accords with my view. (Mon. der Blastoideen, p. 378). In 1868 I discovered the five small pores at the apical extremities of the ambulacral grooves. (This Jour., II, xcvii, p. 353, and Annals Nat. Hist., IV, vol. 4, p. 76). In general it is difficult to see these pores, but if a silicified specimen, which has been fossilized in a calcareous matrix, be placed in an acid for two or three minutes, the acid cleans them out and they then become distinctly visible. I believe these to be the pores through which the ovarian tubes passed outward along the grooves to the pinnulæ. There are thus, sixteen apertures in the apex of Nucleocrinus,-ten spiracles, five ovarian orifices, and one oro-anal aperture. There are no true radial plates. The whole of the test with the exception, perhaps, of the ambulacra belongs to the perisomatic system.

8. On the occurrence of Embryonic forms among the Paleozoic Echinoderms.

MV

Fig. 8. Bipinnaria asterigera Sars, (copied from Müller). a, the stomach; b, part of the body of the larva; c, ambulacral centre, position of the permanent mouth, in this stage not open; d, one of the five ambulacral canals; e, sand canal, f, madreporic plate; m, entrance into the stomach; o, oesophagus; p, larval mouth or pseudostome; r, oesophageal ring; v, vent. 9. Ideal figure described below. 10. Codonites stelliformis, oblique view to show both body and summit. 11. Summit of fig. 10.

No proposition in Natural History has been more clearly demonstrated than this:-That, in general, the paleozoic animals resemble, both in external form and internal structure, the embryonic stages of those of the same class at present existing. Prof. Agassiz has long taught in his lectures and various publications, that this is especially observable in the Echinodermata. Judging from the figures and descriptions of Müller, Agassiz, Thomson, Carpenter and others, I should say, that in this class, the most striking resemblance is that which occurs between the adult stages of the Cystidea, Blastoidea, and Crinoidea, on the one hand, and the embryonic Star-fishes on the other. The structural character that has the most important bearing on the subjects discussed in these notes, is, that in all four of these groups, the mouth is situated in one of the interradial areas,not in the ambulacral center, as it is in the adult forms of the existing Echinodermata.

In Bipinaria asterigera Sars, according to Müller, the digestive cavity is a sub-globular sack without any extensions into the rays, as there are in the adult Star-fishes. The oesophagus, fig. 8, o, is a fleshy, consistent tube, with a large mouth or pseudostome, p. It passes through the wall of the stomach by an opening somewhat smaller than the mouth, and situated in one of the interradial spaces at m. The madreporic plate, f, and sand canal, e, the latter holding the convoluted plate (when it

occurs), are situated above the orifice, m, and between it and the ambulacral center, c. The circular space at c, is undoubtedly the homologue of the central space in the apex of Nucleocrinus, figs. 3 and 5, and of Codonites, figs. 10 and 11. It is also the position of the mouth in the adult Star-fish; but in the larval stage it is completely closed by the soft external skin and sarcode of the body. In the fossils it is also closed, but by an integument of thin calcareous plates. The Bipinnaria is nourished by minute particles of matter diffused through the water, and drawn into the digestive sack through the mouth and oesophagus by the action of interradial cilia. I believe that all the fossil Crinoidea, Blastoidea and Cystidea, ingested their food in this way, and without any aid whatever from the arms or pinnulæ.

Perhaps there is no embryologist who will not admit, that it is possible for an animal like Bipinnaria to develope organs of reproduction and propagate its species, none of its other parts making any farther advance. Such an animal, with some slight modifications, would not be very widely different from a paleozoic Crinoid. If the sarcodic body wall were to be consolidated into a thin calcareous integument, with the mouth even with the surface, the swimming appendages aborted, and the vent closed up, it would resemble the cup of an Actinocrinus, fig. 9, a. The lateral orifice would then be both mouth and vent, as it is, at first (according to Prof. A. Agassiz, Seaside Studies, p. 125), in the embryo of Asteracanthion Berylinus. The ambulacral canals of Bipinnaria are the homologues, in a general way, of those which are found beneath the vault of Actinocrinus, and extend out into the grooves of the arms. If the ventral perisome of the Crinoid were to be removed (the internal organs remaining undisturbed) the arrangement disclosed would be that represented in fig. 9,—a convoluted plate in the center with the canals radiating from it. The most striking difference is the absence of the oesophageal ring. According to the organization of Actinocrinus there could be no oesphagus at that point, and consequently there is no ring. The convoluted plate represents the madreporic apparatus. sucking feet of the Star-fish, most probably, represent the respiratory tentacles that border the grooves of the Crinoids, but modified into prehensile and locomotive organs. Bipinnaria and Actinocrinus agree in having the mouth in one of the interradial areas, and in the absence of an orifice through the perisome at the ambulacral center. These two characters are embryonic and transitory in the Star-fish, but they were permanent in most paleozoic Crinoids.

The

In Codonites stelliformis (Pentremites stelliformis Owen and Shumard), figs. 10, 11, the ambulacral center, c, is completely AM. JOUR. SCI.-SECOND SERIES, VOL. L, No. 149.—SEPT., 1870.

closed. Five minute grooves radiate out to the extremities of the five angles of the disc. These grooves are identical with those of Pentremites and Nucleocrinus and were occupied by the ovarian tubes. The ambulacral canals of the true Crinoids and of the Star-fishes are represented in a rudimentary condition, in this species, by the hydrospires which open out to the surface through the ten fissure-like spiracles, s. The oro-anal orifice is interradial. C. stelliformis in external form, the interradial position of the mouth, and the closed ambulacral center, resembles Bipinnaria and Actinocrinus, but differs importantly in having its respiratory organs arranged in ten separate tracts, all totally disconnected from each other. It is a lower form than Actinocrinus, which in its turn is lower than Bipinnaria, and yet all three are constructed on the same general plan.

C. stelliformis, although much resembling a Pentremite, is a true Cystidean. Its affinity to Codaster was first pointed out by Dr. C. A. White, who also suggested that it should be assigned to a distinct group. (Bost. Jour. N. H., vol. vii, pp. 486, 487). The main difference between the Cystidea and the Blastoidea is, that in the former the hydrospires do not communicate with the pinnulæ, whilst in the latter the cavities of the pinnulæ and hydrospires are directly connected by the ambulacral pores.

The developement of the recent Crinoid Antedon rosaceus, as described by Prof. Wyville Thomson (Phil. Trans., 1866), pursues a course that could not possibly result in the production of such an animal as Actinocrinus. The pseudembryo, as it is called by Prof. Thomson, is a small ovate organism, with four transverse ciliated bands, a large key-hole-shaped mouth (pseudostome), and a small circular vent (pseudoproct). These orifices are connected by a rudimentary intestine (pseudocele). In this stage there is no trace of radiation, and the mouth, therefore, cannot be said to be interradial in its position.

The nascent Crinoid originates within the pseudembryo, but developes a mouth, vent and stomach, of its own, all quite distinct from those of its nurse. This new, or permanent mouth, is for a short time both oral and anal in its function, but although in this respect it resembles that of Actinocrinus, its position in the center of the ambulacral system, shows it to represent the mouth of the adult Star-fish, while that of Actinocrinus rather homologates with the oral orifice of the Bipinnaria. At no time during its development does the ventral perisome exhibit the structure of that of the paleocrinoids, i. e., no orifice in the ambulacral center, and at the same time one in an interradial space. In the central position of its mouth, and in the possession of an oesophageal ring, Antedon stands

above Actinocrinus in rank, and between it and the adult Starfish. In none of its stages does it resemble a Bipinnaria either in form or in structure.

9. On some of the objections that have been advanced against the views advocated in the preceding notes.

In all the known species of the existing Echinodermata, the mouth is situated in the center of the ambulacral system, and it is contended that this fact proves that such must have been its position also in the paleozoic forms.

This reasoning is not strictly logical. It is true that in the known existing species, the mouth is in the center, but it does not certainly follow that it is so in all the Echinodermata, living and extinct. Whether it be so or not in any particular fossil species whose structure may be under investigation, is a question of fact which can only be positively determined by direct observation of specimens. On appealing to these we find that, in a large proportion of the fossil forms, there is no aperture in the perisome at the ambulacral center. It also becomes evident by the comparison that, in general, the paleozoic species resemble the embryonic stages of some of the recent Echinoderms, and that in these, (Bipinnaria for instance), the mouth is interradial. Rules such as is relied on in this case, afford a certain amount of presumptive evidence, which, however, cannot prevail against material and visible facts. When we can see clearly that there is no aperture in that point, in the vault of a Crinoid, beneath which we know the ambulacral center is situated, it is perfectly useless to supply one by deduction.*

The second objection is, that many of the fossils have a Platyceras attached to them, in such a position as to cover the aperture which I call the mouth, and under such circumstances as to induce the belief that it lived parasitically on the Crinoid. The only answer I can make to this is that, admitting the facts, we must suppose that space was left for a stream of water to pass under the edge of the shell, into the mouth of the Crinoid. In general, where one animal lives parasitically upon another, it does not destroy its host. Some of the gasteropods of the Devonian and Carboniferous ages, were carnivorous, as is proved by the bored shells and Crinoids that are occasionally found. I have seen a number of such specimens, and several

*The position of the ambulacral center may thus be found. When the mouth is eccentric, the ambulacral tubes usually converge to the center of the vault. But when the mouth is central, we first find the azygos interradius, in general easily recognized by its possessing a greater number of plates than do any one of the other four interradii. On the opposite side of the fossil is the azygos arm. The ambulacral center is always situated between this arm and the mouth, never on the side of the mouth toward the azygos interradius.