268 ZOOLOGY 



ally backwards and forwards in such a way as to move the 

 corpusculated fluid forward in the dorsal part of the body and 

 backward in the ventral. At the same time certain dilator 

 muscles attached to the rectum cause this tube to expand and 

 take in through the anus a certain amount of water, which is 

 subsequently ejected again. This rhythmical taking in and 

 qj acting of water has probably a respiratory significance, and the 

 process has been termed anal respiration. It has been observed 

 in numerous Crustacea amongst various classes, e.g. in Argulus 

 and Caligus amongst the Copepoda; in Daphnia, Moina, and 

 the larvae of Apics amongst the Phyllopoda ; in Gammarus and 

 Asellus amongst the Arthrostraca ; and in the Zoaea-larvae of 

 Macrura and Brachyura. The interchange of gases between 

 the circulating medium and the surrounding water may, in the 

 case of Cyclops, where the integument is thin, take place all 

 over the body, but in those animals whose cuticle is thick it 

 may reasonably be supposed to take place more readily through 

 the thinly-hned rectum. 



The kidney consists of the typical Entomostracan shell- or 

 maxillary gland ; it is a simple tube ending bHndly at one end, 

 and opening to the exterior on the second ramus of the second 

 maxiUa at the other. The functions of the kidney are believed 

 to be partly taken over by the walls of the intestine, which 

 excrete granules supposed to be urinary. The larva of Cyclops 

 is said to possess an antennary gland. 



The nervous system consists of a supra-oesophageal mass 

 which gives off nerves to the eyes and to the antennules ; of 

 two circum-oesophageal cords from which the nerves to the 

 antennae arise ; and of a single ventral cord which extends to 

 the fifth thoracic segment, where it bifurcates, and continues 

 through the abdomen as two cords, which ultimately end in 

 the furcal processes. The ventral cord presents no marked 

 distinction into ganglionic and interganglionic regions. 



Besides the numerous sensory hairs, each supplied with 

 nerve fibres, the median frontal eye, divided into three parts, is 

 the sole sense organ. 



The Copepoda are dioecious ; in Cyclops the ovary is a 

 median gland situated beneath the first thoracic tergum. The 

 oviduct, which is continuous with the ovary, gives off a series 



