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 Baphnia gives off two stout nerves, which 

 pass forward and almost immediately fuse to form a large optic 

 ganglion, which supplies 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 



17 



