The calcareous crust of the animal is thick, and in the dorsal region capable of great resistance; their members are also of remarkable hardness; but in the smaller species the shell is often thin, and of that crystalline transparency which permits of its digestion and circulation being observed. Many species, which are quite microscopic, contribute colour to the sea—red, purple, or scarlet: such are Grimothea D'Urvillei and G. gregarea.
Before the year 1823, it was not generally supposed that this class of animals was subject to change of shape from the larva condition, and during its progressive development; but about this time, and for some years following, certain able microscopic experiments clearly demonstrated that a minute nondescript kind of animal called the Zoea Taurus, was nothing more nor less than the young of a kind of Prawn as when extracted from the egg. Mr. Vaughan Thomson, by many successive observations, and under the fire of much adverse criticism, satisfactorily established the truth of metamorphic change in many genera, and, in particular, in regard to the common crab (Cancer pagurus); having succeeded in hatching the ova of this species, the product of which were fine Zoeas. That there are variations in the channel of this law of change has been admitted, but that generally a metamorphosis exists, analogous to that of insects, in the various genera of Crustacea, with hardly an exception, has been clearly established.
Fig. 331. Zoea Taurus.
The recorded observations of the eminent naturalist we have mentioned, Mr. Thomson, as well as those of Mr. Couch, of Penzance, Mr. Milne Edwards, and particularly those of the last mentioned, the learned author of perhaps the best work extant on the Crustacea, are referred to as treating most lucidly on this interesting subject.
As an illustration of this metamorphosis, we give figures of the Zoea Taurus in two states, viz., Fig. a, in the first stage; and second, Fig. b, as the animal appeared on the fourth day after the first microscopic examination, and when it resolved itself into a kind of prawn. The drawings appear in Mr. Bell's "History of British Stalk-eyed Crustacea," and were taken by that gentleman from the work of a Dutch naturalist, named Slabber, who made the original observation in the year 1768, and published the result in 1778, from which time the subject had been allowed to fall asleep until revived by Mr. Thomson.
Among the sea-spiders, which have no neck (Cephalothorax), the head gradually disappears in the breast, but the belly remains distinct; the middle of the body is compressed, the shape narrow and graceful. Among the Crustaceans which have neither neck nor shape, the head, the breast, and the belly form only one mass, often short, squat, athletic, and difficult to take, as in Pisa tetraodon (Fig. 332), the four-horned spider-crab.
Fig. 332. Pisa tetraodon.