306 PHYL UM ARTHROPODA. 



structures appear, e.g. the characteristic Cirri ped 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 exceedingly 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. 



On the ventral surface of the abdomen of crabs, Sacculma, one of the 

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

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

 ovoid sac, and is attached about 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 into numerous "roots," which ramify within the 

 body of the crab, and by them the Sacculina obtains nutrition and 

 gets rid of its waste products ; it is practically an endoparasite. The 

 larvae leave the brood-chamber as Nauplii ; they moult rapidly and 

 become Cyprid larvae. These fix themselves by their antennae to 

 young crabs, at the uncalcified membrane round the base of large 

 bristles. The thorax and abdomen are cast off; the structures within 

 the head region contract ; eyes, tendons, pigment, the remaining yolk 

 and the carapace, are lost ; a little sac remains, which passes into the 

 interior of the crab. It reaches the abdomen, and, as it approaches 

 maturity, the integuments of the crab are dissolved beneath it, and 

 the sac-like body protrudes. It appears to live for three years, during 

 which time the growth of its host is arrested, and no moult occurs. 

 Forms allied to Sacculina are grouped together as Rhizocephala. 

 One of them Sesarmaxenos occurs on a fresh-water crab, Sesarma, 

 in the Andamans ; all the rest are marine. 



Second Sub-Class. MALACOSTRACA 



Series I. Leptostraca. Division Phyllocarida. 



Marine Crustaceans of great systematic interest, retaining in many 

 ways the simplicity of ancestral forms, and linking Malacostraca and 

 Entomostraca. The most important genus is Nebalia. 



A bivalve shell covers the whole of the lank -body, except the last 

 four abdominal segments ; the head is free from the thorax ; the eight 

 segments of the thorax are free from one another, and the plate-like 

 appendages resemble those of Phyllopods ; the abdomen has seven 

 segments and a telson with two forks ; the elongated heart extends into 

 the abdomen, and has seven pairs of lateral apertures or ostia. There 

 are both antennary and maxillary excretory organs. Nebalia and its 



