23] UPE HISTORY OF TREMATODES— FAUST 23 



the germinal epithelium of the parthenitae arose from a common type of germ 

 cell. In other words, the germ cells of cercariae and parthenitae are homolo- 

 gous. Stages in the phylogenetic development of the germinal epithelium 

 may be outlined thus: 



1. Germinal epithehum non-localized. Example, Cercaria diaphana 

 (Fig. 79), C micropharynx (Fig. 94). Sporocyst cycle. 



2. Germinal epithelium localized; no mouth or suctorial apparatus. Ex- 

 ample, C. glandulosa (Fig. 67). Sporocyst cycle. 



3. Germinal epithehmn localized; suctorial disc or attachment organ 

 opposite germinal cell mass. Example, C. dendritica (Fig. 87), C. racemosa 

 (Fig. 105). Sporcyst cycle. 



4. Germinal epithehum localized opposite a true oral aperture, with phar- 

 ynx and gut present. Example, C flabelliformis (Fig. 43). Redia cycle. 



5. Germinal epithelium locaHzed and specialized into two sorts of con- 

 jugating germ cells, male and female isogametes. (Theoretical.) 



6. Male germ cells prohferated in numbers from the mass of germinal 

 tissue at the posterior end of the body; female germ cells more highly 

 differentiated. Example, C. gracillima (Fig. 149). Cercaria stage of 

 hermaphroditic cycle. 



7. Germinal cells massed into a small number of speciaHzed glands, called 

 testes and ovaries. Example, C. pellucida (Fig. 18). Cercaria stage of 

 hermaphroditic cycle. 



CERCARIA (general) 



The cercaria is the offspring of the parthenita. It is a highly specialized 

 individual, homologous to the immature redia or the sporocyst. Its speciaU- 

 zation has been accounted for by Ssinitzin (1910:38-56) because of 1) a con- 

 siderable period of free-swimming hfe, during which it acquired a tail, and 2) 

 a change to parasitism in the vertebrate, which was at first facultative, but 

 later became obligatory. There are two types of modified characters to be 

 accounted for in the cercaria, in addition to the original characters common 

 to parthenita and cercaria. The tail, the well-developed muscle complex, 

 the nerves innervating the muscle system, together with the sahvary glands 

 and the sensory papillae — all of these bear evidence of a long period of indepen- 

 dent life. When the organism became parasitic, first ectoparasitic, later 

 endoparasitic, the highly developed muscular suckers with their nerve tracts 

 were further developed, while the stylet organs and cystogenous glands were 

 differentiated. The muscular specialization was of primary importance within 

 the host, while the cyst served to protect the worm during the period of transfer 

 from larval to definitive host. 



The cercaria varies in size, altho it is fairly constant for a particular family 

 or genus. Holostome larvae reach a size of 0.63 mm. in length and 0.35 mm. 

 in width (C. ptychocheilus). On the other hand some of the xiphidiocercariae 

 are much more minute, 0. 18 mm. in length by 0.09 mm.in width.{C. micropharynx). 



