287] NORTH AMERICAN MONOSTOMES 69 



the beginning of the oviduct. These characters are fundamental differences 

 such as are used to separate families and groups and on the basis of these 

 differences the supposition of Barker (1916) seems untenable. 



MONOSTOMUM ASPERUM Vaillant 1863 

 Syn: Monostomulum aspersum Brandes 1892 

 Monostomum aspersum Pratt 1902 



Vaillant described this species as follows: 1.4 to 1.9 mm long by 0.94 

 mm wide, elongated, both ends rounded; dorsal surface convex, ventral 

 flat ; cuticula in the anterior % densely covered with small spines regularly 

 spaced and arranged in alternating rows. Mouth opening round. The 

 digestive system composed of muscular esophageal bulb, followed by a long 

 esophagus which extends to the middle of the body where it bifurcates to 

 form the two ceca which extend to the level of the excretory bladder. The 

 excretory bladder forms a semi-oval sac, which occupies the posterior end of 

 the animal. The genital organs are between the excietory bladder and the 

 ends of the intestinal ceca. The male organs are composed of a curved 

 spinous penis, with a testicle, a vesicula seminalis and a deferent canal. 

 The female organs are wanting or in some individuals incompletely devel- 

 oped and consist only of a granular cell mass. 



Found in transparent sub-epidermal cysts of Siren lacertina. 



In a later paper the same author (Vaillant, 1863a:347) expresses doubt 

 in regard to his interpretation of the sex organs which he says appear to 

 be only poorly developed. 



On the basis of the descriptions of Vaillant, Brandes considered this 

 as a larval form but attempted no further indication for the disposition 

 of the species. Monticelli (1892) enumerates it without any attempt to 

 determine its rightful place. Pratt assigns to it the same place as did 

 Brandes. 



Both accounts of this worm are such that the inference may be readily 

 drawn regarding its larval nature. Neither account gives evidence of any 

 individual in a sexually mature state. The sex organs are poorly developed 

 and no mention is made of a uterus or of eggs which would indicate sexual 

 maturity. The exceptional habitat assigned to this worm by Vaillant which 

 he says is comparable to that of Monostomum faba Bremser is not so rare 

 in the light of the present knowledge of the life histories of distomes and 

 is quite in contrast to what is now known of the monostome life history. 

 Many distomes are known to pass a portion of their life cycle encysted in 

 the skin and superficial layers of the body, while no such stage has yet been 

 found in the cycle of a monostome. The description of Vaillant as well as 

 his figures are so inadequate that it would be hazardous to speculate on the 

 systematic position of this form more than to say on the basis of the digestive 

 system the cuticular spines and what has been given regarding the genital 

 glands that it is probably the immature stage of a distome. 



