40 i PARASITES OF ANIMALS 



attaches itself to the Caligus, and there acquires the adult con- 

 dition. The Polystomida comprise a variety of remarkable 

 genera. I accept this family as the equivalent of Dujardin's 

 first group of trematodes which he termed " Onchobothriens," 

 rejecting only his genus Diporpa, which is a juvenile condition 

 of Diplozoon. In this family Van Beneden includes the genera 

 Calceostoma and G-yrodactylus. In all the polystomes we have 

 a more or less ramified intestine, but the reproductive organs 

 conform to the general trematode type. All are hermaphroditic, 

 the eggs being supplied with filamentary appendages, in some 

 only at one pole of the shell, in others at both ends. The 

 water-vascular system is conspicuously developed. All the 

 species are supplied with prehensile hooks. 



In the Diporpa condition of Diplozoon there are two super- 

 numerary hooks, associated with a dorsal sucker at the centre of 

 the body, and it is by means of these organs that a conjugation 

 between two such juvenile forms is effected. These two indi- 

 viduals become organically united for life, after the fashion of 

 the Siamese twins. After conjugation the sexual organs appear. 

 In Onchotyle appendiculata the lower end of the body merges 

 into a curious appendage, which is placed almost at a right 

 angle with the body itself, and in this way, as Van Beneden 

 justly remarks, the entire animal resembles a little hammer, 

 the resemblance being very much heightened by the circumstance 

 that one end of the appendage is cleft so as to correspond, as it 

 were, with the notch which we employ in the action of nail-draw- 

 ing. The Onchotyle appendiculata was first discovered by Kuhn 

 attached to the gills of a dog-fish (8cillium catulus), but it has 

 since been found ectoparasitically lodged upon other marine 

 fishes. With the Gyrodactylidce I include Van Beneden's genus 

 Calceostoma. The gyrodactyles have been classed with the 

 Polystomidae. Amongst the characters standing out most 

 prominently are those having reference to peculiar hooks which 

 project from the great sucking disk. In Calceostoma this 

 mechanism is reduced to a single horny structure placed at the 

 margin of the caudal sucker in the central line. In some 

 Gyrodactyli the hooks are very numerous. In Gyrodactyht* 

 elegans the caudal sucker supports a pair of large laterally- 

 curved hooks, which are placed back to back in the centre of 

 the disk, being connected at their upper ends by a supple- 

 mentary semi-lunar bar. A series of tentacles serve to increase 

 the prehensile action of the sucker. In many species the males 



