PAEASITES DUKltfG THElB WHOLE LIFE. 261 



Another beautiful trematode, the Octocotyle lanceolata, 

 lives abundantly on the gills of the alosa, and another, 

 the Octobothrium merlangus, on those of the whiting. 

 The gills of the Mustelus vulgaris regularly bear another 

 species resembling a leech, but instead of a single sucker 

 there are six ; this is the Onchocotyle appendiculata. 



The bladder of frogs lodges a very beautiful and large 

 trematode which has lately been studied by many 

 naturalists, the Polystomwm integerrimum. Many obser- 

 vations remain to be made on the different phases of the 

 existence of this parasite. Its organization is known, 

 and it has been seen to lay large and beautiful eggs, but 

 its movements have not been observed before its en- 

 trance into the bladder. 



This Polystomum of the frog — and it is no doubt 

 the same with the species Polystomum ocellatum which 

 inhabits the mouth of the European tortoise (Emys 

 Europsea) — lays eggs only in winter, and the eggs of the 

 young ones do not seem to produce more precocious 

 embryos than those of the adult. The embryos are 

 ciliated, unlike those of many of the ectoparasite worms. 

 They much resemble the gyrodactyles, especially by their 

 bristles ; and like these, they inhabit the cavity of the 

 mouth before they migrate into another organ. We may 

 even ask if these singular gyrodactyles, so peculiar in 

 many respects, are not the larval forms of trematodes 

 allied to the polystomum. 



Several important works have lately appeared on 

 the Polystomum integerrimum, by Mons. Stieda in 1870, 

 by Mons. E. Zeller and Mons. Willemoes-Suhm in 

 1872. 



The gyrodactyles, which we have just mentioned, are 



