688 



VERMES CLASS PL A TYHELMINTHES. 



in Central Europe than in England, is the cleft-headed or Broad Tape- worm 

 (Bothriocephalus latus), which is said to attain a length of 30 ft. The 

 head has neither hooks nor protuberant suckers, but is furnished on each 

 side with a deep groove (Fig. 12, c). The intermediate hoats of these worms 

 and of their allies, which live in water-birds, are fresh-water fish of various 

 kinds, and an interesting fact connected with their development is the ability 

 of the larvae to live for a while an independent life, swimming about in water 

 by means of a set of vibratile hairs or cilia with which they are furnished. 



It must not, however, be supposed that man is only infested with these 

 parasites in their sexual stage. There is a kind, called Tania echinococcus, 

 found in dogs, which scatter the eggs broadcast with their dung ; and these, 

 if swallowed by man in connection with water or unwashed salad, are con- 

 verted into embryos, which ultimately, making their way to the liver, give 

 rise to cysts of large size, and often cause the death of the patient. 



FLUKE - WORMS SUB - CLASS TREMATOPA. The Flat - worms belonging 

 to this section are characterised by having smooth (not ciliated), 

 more or less leaf-like, shortish, unsegmented bodies, provided with 

 two or more suckers, by means of which they adhere parasitically to 

 the hosts they infest. According to the number of the suckers, these 

 worms are divided into two sections, the Polystomece and the Distomece. 

 The Polystomere have two suckers in the fore part of the body, and one 

 large one or several smaller ones at the hinder end ; not infrequently, 

 too, they are armed with hooks for clinging. This rich equipment 

 of organs for adhesion is explained by the fact that these creatures live 

 mostly as external parasites upon various kinds of fish, and are thus 

 constantly exposed to the danger of being washed from their moorings 

 by the water. Their development also takes place directly and without 

 any such curious metamorphoses as obtain in many of the entoparasitic 



worms. The following species 

 may be cited as examples of 

 this order : Cydatella anneli- 

 dicola found upon annelidan 

 worms ; Trochopus tubiporus 

 on the Gurnet; and Aspido- 

 gaster conchicola, which lives 

 in the pericardial cavity of 

 the fresh-water mussel. Per- 

 haps the most remarkable 

 form of all is the one known 

 as Diplozoon parddoxum, the 

 double-worm, which frequents 

 the gills of minnow, gudgeon, 

 etc. The young of this worm 

 is ciliated and leads a free 

 swimming life. It is fur- 

 nished with two eye-spots and with a small ventral sucker and a dorsal 

 papilla. In this stage it is known as the Diporpa. After a time the Diporpce 

 settle down upon the gills of their host, and two of them approaching each 

 other unite, the one seizing the papilla of the other with its sucker, then 

 with a dexterous twist the second similarly gets hold of the papilla of the 

 first, and, growing together, the two become inseparably united (Fig. 13,/). 



Fig. 13. 



a-e, LIFR HISTORY OF THE LIVER FLUKB. 

 Free swimming larva ; 6, cercaria larva ; c, redia 

 larva ; d, young fluke ; e t adult fluke ; / Diplozoon 

 paradoxum. 



