306 PHYLUM ARTHROPODA. 
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 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, Sacczlina, 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 Saccz/7a obtains nutrition and 
gets rid of its waste products; it is practically an exdopfaraszte. The 
larvee leave the brood-chamber as Nauplii; they moult rapidly and 
become Cyprid larvae. These fix themselves by their antennz 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. ManacosTRaca 
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 Vebal/za. 
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. Veda/éa and its 
