DAPHNIA PULEX MULLER 1 19 



blind sacs or cceca. There is no differentiation into stomach 

 and intestine, so that the simple midgut performs both functions. 

 The hindgut or rectum is restricted to the last or anal somite. 

 The anus opens at the end of the trunk under the base 

 of the furca, but morphologically it is dorsal to it (almost 

 terminal). 



Excretory system. The excretory system consists of a 

 pair of maxillary (or shell) glands. These glands are modified 

 nephridia. They are situated in the wall of the shell in the region 

 below the heart and consist of a coiled tube, one end of which 

 terminates in a little blind sac representing a modified nephro- 

 stome, while the other end opens to the outside on the ventral 

 surface of the sixth cephalic somite. The glands are called max- 

 illary because in other Entomostraca the sixth cephalic somite 

 has the second pair of maxillae for appendages. 



Circulatory system and body cavity. The circulatory 

 system of Daphnia is an open one. The heart has the shape of an 

 ovoid and is situated dorsally to the midgut and in front of the 

 brood-pouch. It is composed of a few striated muscular cells 

 and has one pair of ostia or side openings through which the 

 blood returns to the heart. There are no blood-vessels. The 

 pulsation of the heart throws the blood forward and backward 

 into the body cavity where it flows between the various organs 

 until it reaches the branchial lacunae of the gills. Here it is 

 oxidized and returns to the heart through the ostia. It is there- 

 fore probable that the heart receives some "venous" blood be- 

 sides the "arterial" blood which comes from the gills. The 

 blood is colorless and contains amoebocytes. 



Respiratory system. The so-called epipodites of the 

 thoracic appendages are developed as gills. The animal keeps 

 up a constant current of water inside its shell by a continuous 

 beating motion of the thoracic limbs. 



Nervous system and sense organs. The nervous system 

 of Daphnia is more or less characteristic of several orders of 

 Entomostraca. The brain or supracesophageal mass consists 



