The Zoëa form of larva differs from the perfect prawn or crab in the absence of the middle portion of the body and its appendages. The mandibles have no palpi, the maxillipeds or foot-jaws are used as feet, whereas in the mature form they serve as jaws. Branchiæ are either wanting or rudimentary, respiration being principally effected through the walls of the carapace. The abdomen and tail are destitute of articulate appendages. The development of Zoëa into the perfect animal has been well described by Mr. Spence Bate[27] in the case of the common crab (Carcinus mænas).
All crabs, as far as we know, with the exception of a species of land crab (Gegarcinus), described by Westwood, pass through a stage more or less resembling that shown in Fig. [38]. On the other hand, the great group of Edriopthalma, comprising Amphipoda (shore-hoppers, &c.) and Isopoda (wood-lice, &c.) pass through no such metamorphosis; the development is direct, as in the Orthoptera. It is true that one species, Tanais Dulongii, though a typical Isopod in form and general character, is said to retain in some points, and especially in the mode of respiration, some peculiarities of the Zoëa type; but this is quite an exceptional case. In Mysis, says F. Müller,[28] “there is still a trace of the Nauplius stage; being transferred back to a period when it had not to provide for itself, the Nauplius has become degraded into a mere skin; in Ligia this larva-skin has lost the traces of limbs, and in Philoscia it is scarcely demonstrable.”
The Echinodermata in most cases “go through a very well-marked metamorphosis, which often has more than one larval stage.... The mass of more or less differentiated sarcode, of which the larva, or pseud-embryo, as opposed to the Echinoderm within it, is made up, always carries upon its exterior certain bilaterally-arranged ciliated bands, by the action of which the whole organism is moved from place to place; and it may be strengthened by the super-addition to it of a framework of calcareous rods.”[29] Müller considered that the mouth and pharynx of the larva were either absorbed or cast off with the calcareous rods, but were never converted into the corresponding organs of the perfect Echinoderm. According to A. Agassiz, however, this is not the case, but on the contrary “the whole larva and all its appendages are gradually drawn into the body, and appropriated.”[30]
Fig. 39.—Larva of Echino-cidaris, seen from above ✕ 6/10 (after Müller).
Fig. [39] represents the larva of a sea-egg (Echino-cidaris) after Müller.[31] The body is transparent, shaped somewhat like a double easel, but with two long horns in front, which, as well as the posterior processes, are supported by calcareous rods. This larva swims by means of minute vibratile hairs, or ciliæ. It has a mouth, stomach, and in fact a well-defined alimentary canal; but no nerves or other internal organs have yet been discovered in it. After swimming about in this condition for a while, it begins to show signs of change. An involution of the integument takes place on one side of the back, and continues to deepen till it reaches a mass or store of what is called blastema, or the raw material of the animal body. This blastema then begins to change, and gradually assumes the form of the perfect Echinoderm.[32]
