II INTERNAL ANATOMY OF CLADOCERA 43 
intestine is usually straight, but in Lynceidae and in some 
Lyncodaphniidae it is coiled (e.g. Peracantha, Fig. 14). 
In Leptodora the alimentary canal is altogether remarkable ; 
the oesophagus is a long and very narrow tube, which runs back 
through the whole length of the thorax and joins the mid-gut in 
the third abdominal segment. The mid-gut is not differentiated 
into stomach and intestine; it has no diverticula of any kind, 
and runs straight backwards to join the short rectum a little in 
front of the anus. 
The heart is always short, and never has more than a single 
pair of lateral openings; it is longest in the Sididae, which show 
some approximation to the Phyllopods in this, as in the 
sheht degree of difference be- 
tween their anterior and 
posterior thoracic limbs. The 
pericardium hes in the one or 
two anterior thoracic segments, 
dorsal to the gut. From the 
heart the blood runs forwards 
to the dorsal part of the head, 
and passes backwards by three 
main channels, one entering each 
side of the Careee while the Fic. 14.—Peracantha truncata, female, 
third runs down the body, x 100. Oxford. 
beneath the alimentary canal 
to dilate into a large sinus round the rectum. This ventral 
blood-channel gives a branch to each limb, which forms a con- 
siderable dilatation in the epipodite, the blood from the limb 
returning to the pericardium by a lateral sinus. From the 
rectum a large sinus runs forwards to the pericardium along 
the dorsal wall of the body. The blood which enters each half 
of the carapace is collected in a median vessel and returned 
through this to the pericardium. 
Those spaces between the viscera which are not filled with 
blood are occupied by a peculiar connective tissue, consisting of 
rounded or polyhedral cells, charged with drops of a fatty material 
which is often brightly coloured. 
The reproductive organs are interesting because of the 
peculiar phenomena connected with the nutrition of the two 
kinds of eggs. The ovaries or testes are epithelial sacs, one on 
