ARTHROPODA 257 
digestive tube passes straight through the body, with hardly 
any change of diameter, to terminate in the anus in the post- 
abdomen. The only structures which open into it are a pair 
of small curved caecal processes which are given off near the 
anterior end, and which are usually regarded as liver diver- 
ticula. 
The heart, which is much shorter than is usual with Crust- 
acea, is correlated with the small size of the animal. It con- 
sists of an oval sac, the muscular nature of whose walls is very 
evident. The sac is suspended in a pericardium which con- 
tains blood; this blood enters the heart through a single ostium 
on each side, and is forced out by the rhythmic contractions of 
the organ through an anterior opening. Although there are 
no blood-vessels with distinct walls, the blood follows a definite 
course, flowing in channels through the various parts of the 
body and shell. The blood contains amoeboid corpuscles. 
A coiled gland, which ends blindly at its inner end, opens 
to the exterior in the region of the second maxillae (Fig. 153). 
This is termed the shell- or maxillary gland, and it is the 
characteristic nitrogenous excretory organ of the Entomostraca, 
as opposed to the antennary gland of the Malacostraca. In 
Estheria the gland terminates in a vesicle, the walls of which 
are lined by flat epithelial cells, and it has been suggested 
that this may represent a portion of the primitive coelom, just 
as does the vesicle at the inner end of the nephridium in 
Peripatus. The larvae of some Phyllopods possess an anten- 
nary gland as well as a shell gland, but this disappears before 
the adult stage is reached. 
Many species of Daphnidae, e.g. Sida, have also a neck gland ; 
these animals swim on their backs, and the neck glands secrete 
a sticky substance which enables them to attach themselves to 
foreign bodies. 
The brain in Daphnia gives off two stout nerves, which 
pass forward and almost immediately fuse to form a large optic 
ganglion, which supples the compound eye; it also gives off 
nerves to the simple eye, to a curious sense organ composed of 
an aggregation of ganglion cells in the neck, and to the first or 
olfactory antennae. <A pair of circum-oesophageal commissures 
surround the oesophagus, and the large swimming antennae 
if 
