216 Arthropoda. Glass 1. Crustacea. 



parasitic Copepoda. The genus Aega, for example (Fig. 176, l), which comprises 

 blood-suckers living on the skin of Pish, is only slightly modified ; the second, third, 

 and fourth thoracic legs are indeed provided with hooks, and adapted for pro- 

 hension, hut the animal is able to move freely about, and is furnished with large 

 eyes ; sexual dimorphism is not pronounced. More adapted to the parasitic life 

 is the clumsy Cymoihoa,* related to Aega (Fig. 176, 2—3), with or without small 

 eyes, with seven pairs of hook-bearing appendages ; living in the mouth and gill- 

 cavity of Fish. Still more modified are the Bopyridae {e.g., Bopyrus), 

 parasitic in the gUl-cavity of Prawns and other Decapoda ; the females are 

 symmetrical, without eyes, and with tiny hook-bearing appendages (Fig. 177, 1) ; 

 the segments of the wide thorax are immovably united : the males possess a 

 more normal Isopod form, but they are of a very small size (dwarf males), and 

 fixed to the abdomen of the female. The females of the Entoniscidee 

 {Entoniscus, etc.. Fig. 177, 2—3), parasitic in certain Crustacea are almost, or 

 entirely apodous, and altogether very remarkable in form; the pigmy males 

 are relatively normal in structure, although somewhat reduced in size. The 

 larvae of the parasitic Isopoda always exhibit a normal isopod foiin, and are 

 free-swimming. 



5. The Tana i das (the genera Tandis, Apseudes, etc.), form a small 

 division of the Isopoda, which differs in several respects from the foregoing, and 

 approaches the Mysidae and Oumacea. The organisms belonging here have only 

 six free thoracic segments, for the first two of these (not the first only, as in most 

 Isopods) are fused with the head. There is a smaU carapace, united 



Fig. 178. Apseudes Latreillei, 2, 3, 4, 8 second, third, etc. thoracic appendages ; 

 A-^ — A^ first and second antennse; ex exopod of the second thoracic appendage; ffg sixth 

 abdominal appendage ; eye. — After Sars. 



dorsally with the two segments, which are fused on to the head, whilst its lateral 

 part is free, and just as in the Mysidse, its inner membranous surface has a 

 respiratory function. Below the carapace, on each side, is the soft e p i p o d 

 of the first thoracic appendage, which keeps up a current of water just as 

 in the latter. The eyes are on short, fixed stalks, clearly marked off from 

 the rest of the head. The exopod of the second antenna is sometimes present. 

 The second and third thoracic feet, of which the first paii- is modified 



* An interesting observation has been made for Cymothoa and a few other parasitic 

 Isopoda; unlike aU other Malacostraca, they are hermaphrodite. Dming- 

 youth the individuals function for a time as males ; the female genitalia are only 

 developed later, when the male organs atrophy. 



