STRUCTURE OF CLINOSTOMUM 215 



laries. In Allocreadium isoporum (Looss, '94, fig. 15) a collect- 

 ing vessel can be recognized which reaches the first body third 

 and there receives the capillary vessels. In Gorgodera cygnoides 

 (Looss, '94, fig. 125) a collecting vessel runs the whole length of 

 the body and at its anterior end meets a vessel which runs back- 

 ward a short distance before the connecting vessel from the cap- 

 illaries meets it. This might be considered as a short recurrent 

 vessel. ' In Distomum echinatum (Looss, '94, fig. 191) a fully 

 developed collecting vessel meets a still longer recurrent vessel. 

 In Harmostomum leptostomum (Looss, '94, fig. 113) the collect- 

 ing vessel is fully developed and the recurrent vessel runs nearly 

 to the posterior end before the two vessels enter it from the capil- 

 laries. Finally in Cotylaspis (Osborn, '04, fig. 26) the recurrent 

 vessel as well as the collecting vessel, is fully developed, the cap- 

 illaries discharging into a canal which is a forward bend of the 

 recurrent vessel. We see from this brief survey of these different 

 forms that, with the gradual development of both collecting and 

 recurrent vessels, an increasing interval is interposed between 

 the capillaries and the exterior. The structure of these two 

 vessels is entirely different, one being entirely destitute of cil- 

 iary apparatus and furnished with muscular tissue, the other being 

 ciliated and devoid of muscle. Clinostomum has its place among 

 the forms with complete collecting and recurrent vessels, and in 

 addition possesses the remarkable system of branches derived from 

 the collecting vessel. 



It would be possible to find a series of forms showing reciprocal 

 developments of excretory collecting vessel and bladder. Thus 

 in Stichocotyle there is no bladder and the collecting vessels are 

 very large; in the closely related Cotylaspis the collecting vessels 

 are narrow tubes and there are two well developed excretory 

 bladders which are rhythmically pulsatile. 



While noting that the parts of the excretory system thus exhibit 

 a series of degrees in the development of complexity of organiza- 

 tion we must not forget that this series is found not in a group of 

 genetically related animals but among forms which are widely 

 separated in the system. On the other hand when we examine 

 forms which are closely related we find great differences. Thus 



