CRUSTACEA. 241 
character and disguising the segmentation. The simple 
annelid eyes are replaced by the compound eyes. 
In the mesodermic organs there are important modi- 
fications from the annelid type. The simple circular and 
longitudinal muscles of the body-wall become largely broken 
up into segmental muscles and limb-muscles, At the same 
time the perivisceral part of the ccelom is replaced by the 
enormously developed hzmoccele or blood-space, the actual 
body-cavity of an arthropod being a venous blood-space 
communicating directly with the heart. The paired 
nephridia or excretory organs are replaced gradually 
within the sub-phylum by excretory organs of another type. 
The nephridia are still present in Perzpatus, but the coxal 
glands of Avachnida, and the shell-gland and green-gland of 
Crustacea, are usually supposed to be much modified 
nephridial organs. Malpighian tubules appear in Jusecéa, 
Arachnida and Myriapoda, Lastly, a centrolecithal type 
of segmentation appears to be characteristic of the Arthro- 
poda. 
The Arthropoda have five classes— (1) Crustacea, (2) 
Protracheata, (3) Myriapoda, (4) Insecta, and (5) Arach- 
nida. 
Crass I.—CRUuUSTACEA. 
The Crustacea are typically aquatic and breathe by gills. 
They have two pairs of antenne or feelers on the head. 
The first five segments are aggregated together into one 
mass, termed the head, and a number of the other segments 
may form a thorax and abdomen. The appendages are 
typically biramous and used for swimming, but more or 
fewer are modified into legs and jaws. The Crustacea are typi-— 
cally marine and the lower marine types have a free nauplius 
larva. This larva is pelagic and has a dorsal shield} an 
unpaired eye and three pairs of swimming appendages 
round the mouth. The first is uniramous and becomes 
the antennules; the second and third are biramous and form 
the antenne and mandibles. The nauplius, like the trocho- 
phore, grows into the adult by elongation of the hind-end of 
the body and production of fresh segments. In the higher 
Crustacea, with much yolk in the egg, a stage comparable 
to the nauplius is passed through in the egg. 
M. 17 
