it 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 

 slight degree of difference be- 

 tween their anterior and 

 posterior thoracic limbs. The 

 pericardium lies 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 carapace, while the FIG _ lL _ 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 



