414 A2TUEOPODA. 



segmented. This ganglionic mass corresponds to the brain and 

 ventral cord and gives off all the nerves. In the higher Crustacea, 

 a distinct brain and ventral ganglionic chain, which is visually 

 elongated and of very varied form, as well as a rich plexus of 

 visceral nerves and ganglia of the sympathetic system are always 

 present. 



Of sense organs, eyes are the most widely distributed. They 

 may have the form either of simple eyes (paired or unpaired), or 

 compound eyes with smooth or faceted cornea ; in the latter case 

 they are often placed on movable stalks, which are attached to the 

 lateral regions of the head. Auditory organs are also present usually 

 in the basal joint of the anterior antenna, rarely in the caudal plate 

 at the posterior end of the body (Jfysis). The delicate hairs and 

 filaments of the anterior antenna are probably olfactory organs. 



The digestive canal is, as a rule, straight, extending from the 

 mouth to the anus at the posterior end of the body. In the higher 

 forms the oesophagus is usually dilated in front of the mesenteron 

 (rnidgut) into a stomach or crop, which is armed with chitinous 

 plates. The mesenteron is provided with simple or ramified 

 hepatic coeca. 



Excretory organs. The so-called shell glands of the lower 

 Crustacea are regarded as urinary organs, as are also the glands 

 opening at the base of the posterior antenna in the Malacostraca. 

 In the Entomostraca the latter are only preserved during larval life. 

 Short tubes, which correspond to the Malpighian tubes of the 

 Tracheata, may also be present on the rectum (Amphipoda). 



The circulatory organs present every possible degree of perfection, 

 from the greatest simplicity to the highest complication of an 

 almost closed system of arterial and venous vessels. The blood is 

 usually colourless, but is sometimes green or even red, and as a 

 rule contains cellular blood corpuscles. 



Respiratory organs are either entirely* wanting, or are repre- 

 sented by branchial tubes on the thoracic or abdominal appendages. 

 In the first case they are often contained in a special branchial 

 cavity at the sides of the cephalothorax. 



Generative organs. With the exception of the hermaphrodite 

 Cirripedia and Isopoda, all Crustacea are of separate sexes. The male 

 and female generative organs usually open on the boundary of the 

 thorax and abdomen, either on the last or the antepenultimate 

 thoracic ring, or on the first abdominal segment. The two sexes 

 are very often distinguished by a number of external characteristics. 



