No. I.] SPHYRANURA OSLERI. 21 



control their pulsations. These are effected by the muscular 

 fibres which line the bladders, and which, like the parenchyma- 

 tous muscles, are provided with oval nuclei. 



Contraction of both bladders was rarely observed simulta- 

 neously ; nor did there appear to be any constancy in the rate of 

 pulsation, at any rate under the artificial conditions of observa- 

 tion. It was observed that the dilatation of the vascular trunks, 

 resulting from pressure under a cover-glass, was accompanied 

 by prolongation of the interval of pulsation from half a minute 

 to a minute and a half. The systole is somewhat slower than 

 the diastole ; it does not affect the complete obliteration of the 

 cavity of the bladder, but merely reduces its diameter from o. i 

 mm to 0.02 mm. 



From each bladder there runs back towards the caudal lamina 

 a strong lateral stem, which gives off numerous twigs to the 

 lamina, and turns sharply upon itself so as to course forwards 

 towards the head. This it does in close association with the 

 before-mentioned part, being sometimes twisted round it, only 

 parting from it at the contractile bladder, where it runs forward 

 to terminate in the capillaries of the upper lip, first forming an 

 anastomosis with that of the other side. 



The walls of the vascular trunks are highly elastic; whether 

 they are provided with muscular fibres throughout it is difficult 

 to say, although such are undoubtedly present in some parts of 

 their course. An outer coat of elastic or muscular fibrils con- 

 tains here and there an elongated or flattened nucleus, and sur- 

 rounds a hyaline inner coat ; no distinct longitudinal muscular 

 fibres, such as described by Poirier for Distomum clavatum and 

 megnini, are demonstrable. 



Although the walls of the finer excretory capillaries are thin- 

 ner, for they rarely exceed i fi in thickness, it is more diffi- 

 cult to give a satisfactory account of their structure. Only one 

 coat formed of a homogeneous refracting substance appears to 

 be present, and this is often so delicate in the finest tubes as to 

 cause one to doubt the existence of a membrane in fresh speci- 

 mens. After a Sphyranura has been under a cover-glass for 

 half an hour the smaller channels become injected with very minute 

 granules, which in the finest capillaries are arranged in a single 

 row, only accumulating in several rows at the anastomoses. In 

 such very fine tubes the lumen appears to be bounded merely 



