Loxogenes arcanum Nickerson. 281 



small at base and taper but little and are much shorter. The 

 spines too are much smaller shorter and less tapering. They are 

 more numerous. It is not clear that they are related to the 

 papillae: some underlie their bases but others do not clearly do so. 



Nothing can be seen in these sections which throws light on 

 the problem of the origin of the spines. In the case of the Annelids 

 the spines can be traced, as to their origin, to definite setigerous 

 glands which are modifications of the epithelium which secretes the 

 cuticle of the outer surface. But no organ for the production of the 

 trematode spines has ever been detected. The spines in ditterent 

 trematodes take on very definite and characteristic forms, so that 

 we are forced to suppose the existence, at some time in the growth 

 of the animal, of very definite mechanisms for their production. 

 Whatever this mechanism may be it has disappeared without leaving 

 any traces behind it. 



The physiological significance of these papillae and spines is 

 not known. Structures which are apparently the equivalent of the 

 papillae are found in various trematodes, thus Loess (1894), in B. 

 perlatum, and D. isoporum. Stuegis (1897) saw such organs in Di- 

 stomum patellare and called them "cilia", and I saw and reported on 

 similar organs in Phyllodistomum americanum in 1903. Watson (1911, 

 p. 400) mentions the fact that the lining of the male duct in the 

 cestode Gijrodactyhis is "covered with delicate spinules", but says 

 that the vagina is "not ciliated" which I take to mean that these 

 structures are absent there. 



V. Spines in the cuticula of the ventral sucker. 



The cuticula of the ventral sucker is also spinous. Fig. 9 is 

 a view from a transverse section passing in the plane of the ventral 

 sucker. The scale shown in the lower left-hand corner of the figure 

 is 0,01 mm. The cuticula lining the cavity of the organ is 

 directly continuous with that of the general surface of the body. 

 As it folds into the cavity of the sucker the spines accompany it. 

 These spines agree in form and substance with those of the outer 

 surface. The cavity of the ventral sucker is lined with cuticula in 

 all trematodes, but in no other forms do the spines follow into 

 interior of the cavity. This instance gains interest from being a 

 solitary exception to the rule. 



It is not at all easy to account for Jie presence of spines in 

 this situation. Their presence would seem to be distinctly disad- 



