290 PHYLUM ARTHROPODA. 



the cement gland. Some of the structures, e.g. the bivalve shell, are 

 lost ; new structures appear, e.g. the characteristic Cirri ped legs and 

 the shell. Throughout this period, which Dai win called the "pupa 

 stage," there is external quiescence, and the young creature con- 

 tinues 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. '1 hey 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, Sacculina, 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 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, within the body of the crab, inio numerous 

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

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

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

 therefore practically, even at this stage, an endoparasite. The larvas 

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

 Cyprid larvre. 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, Sacculina is always endoparasitic. It appears to 

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

 and no moult occurs. 



Second Sub-Class. MALACOSTRACA. 



Legion I. Leptostraca. 



Marine Crustaceans of great systematic interest, because they retain 

 in many ways the simplicity of ancestral forms, and link Malacostraca 

 to Phyllopods. 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 



