Ou Slichocotyle nephropis Cunninghani, a parasito of tho Ameritaii lobster. 407 



of the i!\cTct«»ry vesicle, whicl» in curly stages uf the ontogeny is a 

 simple median structure. Indeed, Montkjklli ('93, p. 176) has al- 

 ready recognized the real character of these organs from the less 

 complete description of them given by Ciinnixcjiiam. 



The narrow terminal portion of the excretory vesicle which extends 

 to the dorsal pore, is lined by a thick layer of cuticular material, 

 similar in character to that forming the outer covering of the body. 

 It will also 1)0 seen from PI. ."U, Fig. 22, that the peripheral layer ot 

 iinmoditied parenchyma cells of the body wall is reflected inward at 

 the excretory p(»ro and surrounds this portion of the excretory vesicle; 

 hence we not unnaturally tind that the cuticular layer has here pre- 

 cisely the same nature as upon the outer surface of the body. 



CrNNiN(;n.\M says of this part of the system: "At the posterior 

 end the two lateral canals terminate in muscular i)ortions which pass 

 inward behind the intestine and unite to form a single median chamber 

 with thick muscular walls. This chamber opens in the usual way by 

 a jiore on the dorsal surface, close to the end of the body. The 

 rhythmical dilatation and contraction of the terminal chamber is very 

 pronounced.'' The general tojjographical relations are described very 

 clearly in the paragraph quoted, but the statement that the terminal 

 [tortious of the excretory system have thick muscular walls is 

 surely due to a mistake of observation on the part of the author. No 

 indicati(tii of muscle fibres can be seen in sections through the parts 

 under discussion, neither does Cunnin(;iiam's figure furnish any evidence 

 of the existence of such fibres. "The rhythmical dilatation and con- 

 traction of the terminal chamber" I have not observed and in view 

 of the absence of muscle fibres in the walls of the chamber, it is not 

 easy to conceive how such a phenomenon could be brought about. 



The cavity of the excretory vesicles is filled with solid highly 

 refractive bodies (PI. 30, Fig. 17), which are composed of several 

 concentric lamellae and move freely too and fro in the lumen of the 

 vesicle as they are propelled by the general contractions of the body. 

 They are remarkably uniform in size, averaging about 20 // in diameter, 

 and by far the greater number are spherical. Some of the occasional 

 variations in form and size are shown in PI. 30, Fig. 17 c, d, e; the 

 average is represented in a and h. As sliown in these figures, the 

 concentric shells are not all of the same thickness and the surface 

 bounding a layer may be either smooth and even or it may be ir- 

 regular and so present in optical section the appearance which is 

 shown ill Fig. 17 c, (/. 



