« EelmineJätka »
Fig. I of the plate represents a longitudinal section through a whole grasshopper, magnified three times. The cavity of the mouth, M, is not very large. The first segment of the digestive canal extends through the head and thorax, and is composed of two divisions, the narrow csophagus, Oe, and the enormous crop, which is itself formed of two parts, an anterior, Cr, with transverse, and a posterior, Cra, with longitudinal ridges; this last terminates in a narrow portion, P, which corresponds to the proventriculus of other insects. This segment of the alimentary canal is especially characterized by its thick and hard cuticula, which almost completely obscures the underlying epithelium, outside of which, however, there is a powerful coat of muscular fibres of the kind called striated. The cuticula is thrown up into ridges and armed with fine but sharp spines that point backwards. The obvious function of these parts is to grind up the food: they are organs of mastication. The features in question are well shown in Fig. 4 of the plate, which represents a transverse section through the posterior part of the crop, magnified forty-five diameters. The ridges, rid, are small and numerous, and upon them can be seen the little spines, s s, and they are covered by the cuticula, which is very thick. The epithelium does not appear distinctly by this magnification, nor do the longitudinal muscles, L, but the transverse or circular muscles form a very thick layer, muc. C. Where the ridges are transverse, as in the front part of the crop and the cesophagus, we find the transverse muscles less developed and the longitudinal the most powerful
The middle segment of the alimentary canal consists of the large ventricle, ven, known in German as the “ Chylusmagen," with six blind sacks or diverticula appended to its anterior extremity; only one of these, however (Div), appears in the section Fig. 1. In this segment the cuticula is very delicate, but the epithelium undergoes another kind of modification, being thrown up into folds. In the diverticula there are twelve longitudinal folds, the structure and arrangement of which can best be seen in a transverse section colored with hæmatoxiline, Fig. 8, Plate II. The folds are high and thin, and covered by the epithelium, which is everywhere of about the same height. Outside there is a thin layer, muc, of muscular fibres, which do not enter into the composition of the folds. Between the muscles and the epithelium
there intervenes another kind of tissue, called connective, which also extends into the centre of each fold, separating the epithelium of the two sides. These three layers, the epithelium or tunica mucosa, the connective tissue or tunica submucosa, and the muscles or tunica muscularis, are the primary constituents of the digestive canal throughout the animal kingdom. It is always the epithelium which is the active agent of the secretion of the digestive juices as well as of the absorption of the food; hence it is important to make the surface of the epithelium which is to come in contact with the food as large as possible, and it is by the formation of folds of various shapes that this is accomplished.
The third and last segment of the alimentary tract consists of three divisions: first, the ileum, Plate , Fig. I, which is as large in diameter as the stomach, for a part of which it was formerly mistaken; second, the very small colon, which bends forward and upwards (col), and opens dorsally into the larger and horizontal rectum, R. The whole of this segment is traversed by six longitudinal folds of the epithelium, which are broad with flat surfaces in both the ileum and rectum, but narrow and irregular in the intervening colon. Each of the six folds ends at the front end of the ileum in two rounded protuberances, making twelve in all. They have hitherto escaped notice. I propose for them the name of gastro-ileal valves. When viewed from their inner surface they have the appearance indicated by Fig. 7, being rounded in front and gradually fading out posteriorly. They are strongly pigmented. In transverse section, Fig. 10, we recognize the three characteristic layers, the epithelium which alone contains the pigment, the very distinct coat of connective tissue, and externally the muscular fibres. Examined with a higher power, Fig. 6, the epithelial cells will be found filled with round granules of various sizes of brownish color; which are the pigment; moreover, there is a thin but quite resistent cuticula armed with minute spines.
In the ileum we find the cells modified in still another way, as shown in Fig. 5, which is a surface view of part of the edge of one of the folds. The cells in the middle are quite large, but their, size diminishes towards the edge, until at the edge itself they are comparatively diminutive. Fig. 9 is a transverse section through one of the furrows between two folds. F is the furrow; Ep the epithelium, the cells of which are smallest in the furrow. The epithelium rests upon a layer of connective tissue, conn,
which is separated from the muscular layer by a wide interspace, the muscular coat muc. C being attached to the connective only underneath the furrows, where there is also a single bend of longitudinal muscles, L, placed outside the transverse muscles.
Finally in the rectum, as shown in a transverse section, the epithelial cells are differentiated into two kinds, larger ones of the ordinary form, and smaller ones which lie higher up and present a circular outline. In vertebrates this differentiation of adjacent epithelial cells is carried to a great extent, and is especially connected with the development of glands; but the discussion of these is foreign to our present purpose.
In conclusion I will give a summary of our observations: Animals are built up of cells; each cell consists of a protoplasmatic body, a nucleus and a nucleolus; the cells are often grouped together so as to form a single continuous layer, which is called an epithelium; the free surface of such a layer is often covered by a thickened membrane, the cuticula, which is formed by the underlying cells. The cells of an epithelium may be modified, first as to size, second as to form, third, character of the cuticula, fourth, position, size, and shape of the nucleus and nucleolus, fifth, presence of granules or pigment, and sixth, differentiation of adjacent cells into two or more kinds. The epithelium may be modified by the formation of folds and pits of various forms.
NO.'I. THE OCEAN BEACH.
inviting field to the naturalist. A wilderness of rocky hills and sanddunes, bearing no vegetation larger than a scrub oak, and swept by the winds and fogs of the great ocean, it lies, between the Pacific and the Bay of San Francisco, a bare and naked spot in the long wooded coast-line of California.
Yet, here as elsewhere, he who seeks with willing and educated eyes, will not fail to find much to interest him. The sand dunes, the cliffs that border ocean and bay, and the rounded sandstone hills have each their special flora; many species of gulls, ducks and divers, herons, pelicans and cormorants visit the bay and the
marshes in the winter months; and the shores of the ocean and the bay, though poor in species compared with points either northward or southward along the coast, have each their characteristic forms of life.
Pleasant it is, when the clouds have rolled away, and the green hills and bay and ocean lie spread out before us in the glorious sunshine of a Californian spring morning, to leave the city's wooden sidewalks and ramble away to the ocean shore. On such a trip let us now start; let us feast, Barmecide fashion, on the good things of nature, bringing together, as those who feast in such fashion may, all the delicacies of the season to adorn our table.
In the centre of the peninsula, at some distance from any other hill, rises a conical elevation covered with loose sand and surmounted by a tall cross. Around it lie the cemeteries of the city, once far out of town but now becoming gradually surrounded by houses. Near the foot of this "Lone Mountain " we alight from the horse-car and strike out over the sand-hills toward the ocean, stopping, however, to pluck the flowers and to rummage among the old roots of the blue lupine (Lupinus albifrons) and the low bushes of groundsel-tree (Baccharis consanguinea) for the living treasures hidden there in the form of lizards and frogs which have not yet left their winter retreat. A pretty long-tailed black and white lizard (Gerrhonotus multicarinatus Blainv.) is our first prize, soon followed by a small tree frog (Hyla regilla). Another search brings to light a pair of "swifts,” not birds but iguanine lizards (Sceleporus undulatus Harl.) and two or three more tree-frogs.
Our lizards are cold and sleepy, for the warmth of the sun has not yet penetrated to their place of concealment, but after a few minutes in our hands, exposed to the sun's rays, they became more lively, and ran off a short distance. The long slender Gerrhonotus, with his tail trailing along the ground, is strikingly different from the bluff-bodied conical-tailed Scelepori, with their rough-scaled gray backs lined with undulating short stripes of black, and displaying the bright blue of the sides of the abdomen.
Don't touch the Gerrhonotus incautiously when he gets lively, for if you catch him anywhere except just around the neck, he will surely either bite your finger sufficiently hard to make you
drop him, or he will drop off his own tail and leave you a spoiled specimen. The Scelepori are not so fond of throwing off their tails, nor nearly so apt to bite, but in summer time when they are active they are exceedingly hard to catch. They are not called swifts without reason.
What is the color of a tree-frog? Green for the most part, certainly, yet it varies greatly. The black marks upon the back are very distinct in some, quite faint in others, and one of our specimens is brownish, while another inclines to yellow. These fellows we have caught under the bushes are, in one sense, not so “green” as those we will find among the leaves at a more advanced season.
But our low bush furnishes us with insect as well as with reptile life. It literally swarms with “squash-bugs," big black millipedes (Fulus) lie coiled among the decaying leaves, a long brightred Scolopendra hurries away from our invading hand, and hunting spiders tun to and fro in hot haste.
We are now on a rising ground at the edge of the Presidio reservation. Below us, in a little valley encircled by hills, lies a small fresh-water lake known as Mountain Lake, a line of shrubs beyond this marks the course of the stream which runs thence to the ocean, and the view is closed by the cliff-encircled bay outside the Golden Gate, forming the entrance to the harbor of San Francisco. Away on the farther side of the bay is the white lighthouse of Point Bonita, and oceanward a line of breakers marks the dreaded bar.
We hasten down to the lake, hoping to find there, as we have found before, numerous specimens of the fresh-water snails of the genera Limnophysa, Physa and Helisoma, as well as the tiny flat Gyraulus vermicularis Gld. But we have forgotten the thirty inches of rain that have fallen since we visited the spot in September last. The lake that was then so low that we could explore much of its bed, and so clear that we could see the bottom, is now a broad sheet of turbid water, in which the shell-fish are lost to sight. The spot where we picked up the flat-whorled Helisoma in shallow water, full of confervæ is now far from the shore, and the beach, once strewed with the shells of the fresh-water clam (Anodonta wallamatensis, sweet name this), is now lake bottom again.
The yellow water-lilies (Nuphar advena) whose tops and mas