414 



CHARACTERS OF INVERTEBRATE ANIMALS 



the other members of the sub-order, they are burrowing forms 

 found in shallow water. 



4. The sub-order Cumacea includes small shrimp -like forms 

 (fig- 255) which live in shoals, and inhabit fairly deep parts of the 

 sea where the bottom is of sand. The cephalo-thorax is even more 



ANT. 1 



CEPHALOTHORAX 



Fig. 254.— C«wz<z (enlarged), ant. i, Antennule iv-viir, posterior thoracic segments: 1-7, abdominal segments. 



limited than in the Locust-Shrimps, for the last five thoracic seg- 

 ments are not united with it. Some at least of the thoracic limbs 

 possess outer as well as inner branches, and the two compound 

 eyes are stalkless and sometimes fused together. 



Order 2. — Sessile-EYED CRUSTACEA (Arthrostraca) 



The order includes a large number of comparatively small 

 Crustacea, in which, as a rule, only one thoracic segment is fused 

 to the head, and there is consequently no great armour-covered 

 cephalo-thorax, as in a Crab or Lobster. The head-appendages 

 correspond to those of a Lobster, but the thorax possesses only 

 one pair of foot-jaws, its other appendages being seven pairs of 

 legs, none of which are provided with pincers. There are usually 

 six pairs of limbs on the abdomen. The eyes are devoid of 

 stalks, i.e. are sessile 



Two sub-orders are recognized, one containing laterally-flattened 

 animals (Amphipoda), and the other forms which are flattened from 

 above downwards (Isopoda). 



I. The most typical members of the Amphipoda (fig. 255) are 

 springing forms, of which the Sand- Hopper {Talitrus locusta) is 

 a good example. During the summer months this may often be 

 seen in leaping myriads between tide-marks on sandy shores. Its 

 appearance suggests a strongly-curved flattened shrimp. Very 

 similar in character are the species of the genus Gammarus, 

 common in shallow water, both salt and fresh. The curious 



