animal; but all "are arranged in this stellate form because they have one anus in common, out of which excreta, &c., are ejected." The mouth of each animal may be seen like a dot in the "ray."* This is sur rounded by cilia, and when alive, is in reality a very active organ, producing miniature whirlpools in the sea water, and thus obtaining fresh oxygen and food. The common anus forms the centre dot of each * Throughout this book such circulating currents in various animals are frequently mentioned, and they are now particularly adverted to, because it is by imitating them in the machinery of aquaria that the best biological results are attained. In this way, perhaps unconsciously, we do as Nature does, and then find that she, long previously, has done the same thing in a far neater and better manner. SHAPES OF ANIMALS. 253 "star." We have long since learned that the mere magnitudes and even shapes of animals are not the most important characters by which to associate groups, but this fact is nowhere felt in such a degree as when we study the polyzoa and their allies. CHAPTER XV. CRUSTACEA, ECHINODERMS, ANNELIDS, ETC., OF MARINE AQUARIA. ONE of the most important advantages which large aquaria, aerated by constant jets of water, possess, is the great variety of marine animals, vertebrate and invertebrate, which may be kept in them. Not only may they be so selected as that one group shall not harm another, but much of the labour and anxiety likely to occur from the pollution of the water by decomposing food or dead animals, may be prevented by merely including certain omnivorous creatures which will clear away such garbage and consume it as food. Some of the carnivorous mollusca (the whelks, for instance) are useful in this respect, but many of the crustacea are even more so. The latter, also, are more lively animals, and never fail to cause amusement by their grotesque and serio-comic habits. One cannot witness the rude gambols of lobsters and crabs without feeling that the element of humour is not merely a subjective condition of the human mind, but has an objective existence in nature. Of all the useful and interesting scavengers, none is more so than the hermit crab (Pagurus Bernhardus), HERMIT AND SOLDIER CRABS. 255 so called from its occupying an empty shell, as a hermit would his cave. This crustacean is forced to this singular habit by the softness of its abdomen, which does not secrete a hard crust, and therefore Hermit Crab (Pagurus Bernhardus). requires such protection as an empty whelk-shell will afford. Nothing on earth exceeds the shamefacedness of a hermit crab deprived of its shell-not even a bather whose clothes have been stolen! When perfectly accommodated with an empty shell, the hermit crab is most suspicious and wary. It regards everything as an enemy; and even when one of its own kind approaches, you will see it move away, or draw itself within its cave, and close the aperture by means of its large right claw, which is bigger than the other for the purpose. Being as warlike as they are suspicious, a good many fights come off in the tanks between them, insomuch that this species is also called the "soldier crab." The somewhat vulgar practice is now followed of introducing highly-coloured empty tropical shells for the hermit crabs to take up their abodes in, although this has added to the attractive appearance of the bottoms of the tanks. Before moulting these crabs generally leave their old domiciles, and select one so much bigger that they can move about in it. Their increase in bulk is then usually very rapid. Many a contest comes off between hermit crabs, when two of them wish for the same empty shell. Every atom of food rejected by other animals in the tank, and which would otherwise lie on the floor and foul the water, is greedily cleared away by hermit crabs and their allies. The graceful appearance of lobsters when at rest makes them prominent objects. They seem to be in almost a devotional attitude, resting on their huge pincers as the "praying mantis" does on its fore-legs; and in this position are often seen with their faces towards the glass front. Their long, slender, jointed antennæ are thrown backwards, and are in a constant state of motion. The spiny lobster, or "sea-crayfish" (Palinurus quadricornis), is a more attractive marine object than the common lobster, although its flesh is not such delicate food as that of the latter. Its body is covered all over with spines and prickles, and is moreover very brightly coloured. In length this species exceeds any other British crustacea. It is noticeable, however, by the absence of the large and powerful pincers which distinguish the common lobster. The females of this species have spawned both |