376 THE INVERTEBRATA 



Stage becomes parasitic on the gills of a flat fish, deriving nourish- 

 ment from its host by means of suctorial mouth parts. Here it passes 

 into a "pupal" stage in which the power of movement is lost and 

 retrogressive changes have taken place. Presently it regains the power 

 of swimming and leaves the host in an adult copepod stage. In this 

 stage impregnation takes place. The male develops no further, but 

 the female attaches herself to the gills of a fish of the cod family, 

 where by a great development of the genital somite she becomes con- 

 verted into a vermiform parasite, anchored into the host by processes 

 that grow out from her head, and retaining only the now relatively 

 minute appendages of the thorax. 



In Herpyllobius, parasitic on annelids, the female is reduced to a 

 mere sac, drawing nourishment from the host by rootlets and bearing 

 minute males which are also sac-like. 



Xenocoeloma, also parasitic on annelids, is represented in the host's 

 body only by the gonads, which are hermaphrodite, and some 

 muscles, enclosed in a cylindrical outgrowth of the host's epithelium 

 which forms a body wall for the vestiges of the parasite and contains 

 a gut-like prolongation of the host's coelom. 



Class BRANCHIURA 

 Crustacea, temporarily parasitic on fishes; which possess compound 

 eyes ; a suctorial mouth ; carapace-like lateral expansions of the head 

 which are fused to the sides of the first thoracic somite ; an unseg- 

 mented, limbless, bilobed abdomen with a minute caudal furca; and 

 four pairs of thoracic limbs, which are biramous, with usually a 

 proximal extension of the exopodite. 



The members of this group in many respects superficially resemble 

 the Copepoda, with which they are generally placed, but differ from 

 that class in certain important features, notably in the possession of 

 compound eyes, the lateral head-lobes, the opening of the genital 

 ducts between the fourth pair of thoracic limbs, and the phyllopod- 

 like proximal overhang of some of the thoracic exopodites (Fig, 254 B). 



The carp-lice, as the Branchiura are called, are found both on fresh- 

 water and marine fishes. They are good swimmers. The females 

 deposit their eggs on stones and other objects. The larvae differ little 

 from the adult. 



Argulus (Fig. 254), the principal genus, has a pair of suckers on 

 the maxillae and a poison spine in front of the proboscis. A.foliaceus 

 is common on freshwater fishes in Britain and the Continent. 



Class CIRRIPEDIA 



Fixed and for the most part hermaphrodite Crustacea ; without com- 

 pound eyes in the adult ; with a carapace (except in rare instances) as 



