59] LIFE HISTOR Y OF TREMA TODES—FA US T 59 



tion. It is spinose internally (Fig, 20). In the prepharynx region is a unique 

 piercing organ (Fig. 17), probably of ectodermal origin. It is four-lobed and 

 is covered with spines. A rythmic eversion of the organ against the host tissue 

 and redrawal within the pharynx region of the parthenita is a characteristic 

 movement of the redia. Around the muscular pharynx is a ganglion mass 

 consisting of a fibrous matrix and a network of ganglion cells. Behind the 

 head region is a neck-like constriction, and behind the neck is a sacculate 

 body. At the extreme posterior end is a large papilla. The redia. is covered 

 with a non-cellular basement membrane, and imbedded in this superficially in 

 the form of minute tuberosities are the remains of the epidermal nuclei 

 (Fig. 22). 



The walls of the redia are well-supplied with muscular layers, longitudinal 

 and transverse, so that the parthenita is capable of extraordinary distension 

 and contraction, altho it has no specific locomotor organs. 



The germ balls of the redia arise from the matured ova, derived from four 

 cells localized at the posterior extremity of the parthenita. Altho the cells 

 lying next to the wall around this quartet may be potentially germ cells, they 

 take no part in the ordinary proliferation of germ cells (Fig. 22). From these 

 cells arise the germ balls, thru cleavage into 2, 3 and 5 cells, after which cer- 

 tain cells of the ball appear much smaller than the others and grow over the 

 latter, giving rise to the gastrula by epiboly. The young germ balls usually 

 lie en masse behind the gut, while the more advanced cercariae are crowded 

 anteriorly. They appear strangely grotesque, with their pigment eyes and their 

 snout-like bodies oscillating back and forth within the body wall of the par- 

 thenita. 



Aside from the larger size of the body and the trioculate anterior end, 

 Cercaria pellucida might be at first confused with Cercaria urbanensis Cort. 

 While the size and eye-spots are sufficient to separate these two species, a 

 more careful examination shows that there has not been a separation of two 

 species at all, but more correctly two groups of species. The group of smaller 

 species is binoculate and ranges around 0.3 mm. to 0.46 mm. in length by 0.1 

 mm. to 0.16 mm. in width, while the group of larger species is trioculate and 

 averages around 0.5 mm. in length by 0.15 mm. to 0.2 mm. in width. Con- 

 sequently from a description of external characters alone there is no means of 

 separating Glenocercaria lucania Leidy from the Bitter Root species Cercaria 

 pellucida. It is such a problem as this that has caused the writer to believe 

 that there are characters more deeply seated in the larva that will readily 

 set it off from others of the same group. 



Sufficient care in technic makes it possible to bring out very clearly and 

 convincingly the genital organs of the Bitter Root species. Here are characters, 

 constant both in the larva and the adult that readily differentiate these mon- 

 ostome cercariae. These have been described in detail in the section devoted 

 to morphology (p. 45) and need only to be summarized at this point. The 

 median ovary just in front of the excretory bladder opens out thru a short 

 duct at its left (Fig. 18), and after receiving the common vitelline duct, opens 



