248 MARINE INVERTEBRATES 
The muscles are white bundles of fibers, and are in strips, 
which is an indication of power and activity. There are four 
important sets: twisted ventral muscles which bend the tail and 
are particularly large and strong, those which straighten the tail, 
those moving the appendages, and 
those which work the gastric mill. 
The eggs, after being disvharged, 
are attached to the abdominal legs of 
the mother by a kind of cement, or 
they are carried in pouches attached 
to the thorax. In these positions they 
mature, and hatch at different stages 
of development in different species. 
Zoéa of Cancer irroratus, Last stage Some emerge with three pairs of legs, 
before it changes to the megalops con» and are known as Nauplius; some are 
Zoéa, having a carapace and abdominal 
segments, butno abdominalappendages. Anotherstageis Megalops, 
with large stalked eyes. Others are hatched as miniature adults. 
These names were given when the embryo stage was not recog- 
nized and the larvee were thought to be distinct species. 
The growth of the animal is effected by moulting. The Crus- 
tacea are named from the crust-like covering 
which envelops them. Itis a horny material, 
called chitin, in which are deposited particles 
of carbonate of lime, making a rigid envelop 
which would prevent all freedom of motion, 
were it not that there are spaces free from 
lime, and thus flexible joints are left. When 
the animal expands it throws off this hard 
covering and secretes a new and larger one. 
As the time of moulting approaches, the old 
covering becomes loosened, and a delicate — segatops stage of Cuncer 
new one is formed beneath it. The old shell jrrerains. ais change 
splits open across the back just behind the 
carapace, and the soft animal withdraws first its cephalothorax 
and then its abdomen, leaving the cover complete, including even 
the covering of the eyes and the lining of the stomach. The 
