FEEDING, LOCOMOTION AND RESPIRATION 33 



has passed somewhere near the Tanaidacea, some of which still 

 have a maxillary filter, but it seems to be of minor importance to 

 them, and they feed mainly on large particles. Most of the isopods 

 and amphipods have biting mouthparts; the maxillules often have 

 strong sharp spines which help to break up the food, and one or 

 more pairs of legs may be subchelate. This modification is useful 

 for holding food against the mouthparts while pieces are being 

 broken off. Some species, such as the isopod Mesidotea entomon use 

 the subchelate thoracic legs to catch small animals, and can 

 manipulate their prey with considerable dexterity. 



The caprellids (fig. 23) are usually found attached to small 

 colonial coelenterates, and it used to be thought that they fed on the 

 small polyps, but they have been observed to catch copepods as 



Fig. 23. Pseudoprotella phasma. (Amphipoda, Caprellidea) lateral 

 view of male 



they swim by. The caprellid has a sense organ in its antennule 

 which is sensitive to small water currents. When a movement is 

 perceived the caprellid turns towards it with its large subchelate 

 second leg held in a ready position. The actual catching of the prey 

 is done with the large subchela and is probably dependant on the 

 prey being sighted. Once caught the prey is held in front of the 

 mouthparts and small pieces are torn off by the small first pair of 

 subchelae and pushed into the mouth. 



One general feature to be noticed in the amphipods and isopods 

 is that the thoracic legs are not used for swimming. They have 

 become true walking legs, and swimming, when done, is performed 

 by means of the pleopods on the abdomen. In the Isopoda the 

 pleopods are also respiratory organs and are flattened for this 

 purpose, but the amphipods have special flaps, on the inner sides 



D 



