87] LIFE HISTOR Y OF TREMA TODES—FA UST 87 



the vas efferens to the genital pore far cephalad — ^all of these show in common 

 a genital system quite distinct from the usual distome types. 



The similarity of the nervous and genital complexes of larval monostomes, 

 such as Cercaria pellucida and C. konadensis, and the amphistome, Gastro- 

 thylax gregarius, suggest a common origin of certain monostomes and amphi- 

 stomes. 



Among adult distomes there is great variety of structure, yet only in one 

 family, the Schistosomatidae, has the nervous system been fundamentally 

 altered. The structures of the two furcocercous larvae, Cercaria gracilUma 

 and C. tuberistoma, demonstrate their relationship to the Schistosomatidae. 

 The reasons for this beUef are these: 



The characters which distinguish the apharyngeal furcocercous cercariae 

 are as follows: 1) a forked tail (larval character only); 2) paired groups of 

 salivary-mucin glands (larval); 3) absence of an oral suctorial disc, and in 

 its place 4) an invertible suctorial proboscis; 5) an apharyngeal esophagus, pro- 

 vided with glands in the region usually occupied by a pharynx. The nervous 

 system, more deeply seated, is modified by the degeneration of the posterior 

 laterales during early embryonic development, and the fusion of the posterior 

 dorsales with the posterior ventrales about one-third the body length back 

 from !the anterior end (Fig. 150). All of these, except the forked tail and 

 the saUvary-mucin glands, are both larval and adult characters. In addi- 

 tion, the larva of Cercaria gracilUma has several testicular follicles proliferated 

 from the testes-mass at the posterior end of the body. 



Certain of the structures of this group also characterize the larvae of other 

 groups of trematodes. For example, Cercaria cristata La Val. (1855:23; Taf. 

 II, Fig. K), has a bifid tail and apparently lacks a pharynx, but the fact that 

 it lacks an acetabulum probably separates it from the distome f urcocercariae. 

 Among the Gorgorderinae there are apharyngeal cercariae with several testes. 

 However, Ssinitzin (1905:46-51; Taf. I, II) has shown for four Gorgordera 

 species, that the cercariae are characterized in common by 1) a stylet, 2) 

 salivary glands only in the cephalic region of the body, 3) a large glandular 

 excretory vesicle, almost filling the posterior third of the body, and 4) a dis- 

 proportionately large tail, showing the cystocercous relation of the larvae of 

 the group. Moreover, the nervous system of the Gorgorderinae is typically 

 distomate (Zailer 1914:386). 



It may be stated with considerable probabihty that all of the fundamental 

 organs of the f urcocercariae, namely, the apharyngeal esophagus, the multiple 

 testes, and the uniquely modified nervous system, are found in only one 

 family, the Schistosomatidae. All described Schistosomatidae are charac- 

 terized by the absence of a true oral suctorial disc and by the presence of an 

 invertible oral suctorial pouch. They have no pharynx, but in its stead 

 glands that line the wall of the esophagus. Looss has described the modified 

 nervous system of the family in his study of Schistosoma haematobium (1895: 



