CLASSIFICATION OF CRUSTACEA. 275 



The life histor}^ also is similar. A Nauplius is hatched. It has the 

 usual three pairs of legs, an unpaired eye, and a delicate dorsal shield. 

 It moults several times, grows larger, and acquires a firmer shield, a 

 longer spined tail, and stronger legs. Then it passes into a Cypris stage, 

 with two side eyes, six pairs of swimming legs, a bivalve shell, and other 

 organs. As it exerts itself much but does not feed, it is not unnatural 

 that it should sink down as if in fatigue. It fixes itself by its head and 

 antennje, and is glued by the secretion of the cement gland. Some of 

 the structures, e.g., the bivalve shell, are lost ; new structures appear, 

 e.g.. the characteristic Cirriped legs and the shell. Throughout this 

 period, which Darwin called the " pupa stage," there is external 

 quiescence, and the young creature continues to fast. The skin of the 

 pupa moults off ; the adult structures and habits are gradually assumed. 

 At frequent periods of continued growth, the lining of the shell and 

 the cuticle of the legs are shed. In spring these glassy cast coats are 

 e.xceedingly common in the sea. Acorn shells feed on small marine 

 animals. They fix themselves not to rocks only, but also to shells, 

 floating objects, and even to whales and other animals. 



Akippe and Cryptophialiis (with only three or four pairs of feet) live 

 in the shells of other Cirripedes or of Molluscs ; Proteolepas is parasitic 

 in the mantle of other Cirripedes, and like a grub. 



On the ventral surface of the abdomen of crabs, Sacailina. the most 

 degenerate of all parasites, is often found. Its complete history has 

 been beautifully worked out by Professor Delage. It is in shape an 

 ovoid sac, and is attached aVjout the middle of a segment. On the 

 lower surface of the sac there is a cloacal aperture, opening into a large 

 brood chamber, usually distended with eggs contained in chitinous 

 tubes. The brood chamber surrounds the central "visceral mass," 

 consisting of a nerve ganglion, a cement gland which secretes the egg 

 cases, and the hermaphrodite reproductive organs ; of digestive or 

 vascular systems there is no trace. The parasite is attached by a 

 peduncle, dividing up, within the body of the crab, into numerous 

 " roots," which have been compared to the placenta of a mammalian 

 fcetus. The "roots" ramify within the body of the crab, and by them 

 the Sacciilina obtains nutrition and gets rid of its waste products ; it is 

 therefore practically, even at this stage, an eiidoparasite. The larvae 

 leave the brood chamber as Nauplii, they moult rapidly and become 

 Cyprid larvfe. These fix themselves by their antennae to young crabs, 

 at the uncalcified membrane surrounding the base of the large bristles 

 of the back or appendages. The thorax and abdomen are cast off 

 entirely ; the structures within the head region contract ; eyes, tendons, 

 pigment, the remaining yolk, and the carapace are all lost ; and a little 

 sac remains, which passes into the interior of the crab. Eventually it 

 reaches the abdomen, and, as it approaches maturity, the integuments 

 of the crab are dissolved beneath it, and the sac-like body protrudes ; 

 essentially, however, Sacc2tli?ia is always endoparasitic. It appears to 

 live for three years, during which time the growth of its host is arrested, 

 and no moult occurs. 



