268 LOO LOGY 
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 
ejecting 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 Apus 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-lined rectum. 
The kidney consists of the typical Entomostracan shell- or 
maxillary gland ; it is a simple tube ending blindly at one end, 
and opening to the exterior on the second ramus of the second 
maxilla 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 
