38 ILLINOIS BIOLOGICAL MONOGRAPHS [33 



takes place, so that there are two pairs of vessels to proceed forward, an inner 

 and an outer pair. They run forward to a plane in front of the middle of the 

 body, where they unite and run cephalad as a single vessel. A transverse 

 vessel is found posterior to this union; this vessel connects the two sides of 

 the system. Brandes' diagram for the excretory system of D. ahbreviatum 

 (1891, Taf. 39, Fig. 17) differs from this type only in the details and not in 

 the main features. 



The system of Cercaria ptychocheilus (Fig. 49) has a long, attenuate non- 

 muscular bladder dorsal to the posterior genital apparatus. It does not pair 

 but gives off a single median longitudinal vessel, which proceeds forward to 

 the midacetabular region, where it gives off the transverse vessel. The median 

 longitudinal trunk then runs forward to the origin of the ceca. Here it gives 

 rise to three antero-lateral vessel's. These bend outward and backward, with 

 numerous anastomoses, and finally unite with the lateral traces of the trans- 

 verse vessel. The outer reaches of the transverse vessel give rise to many 

 anastomoses which are conspicuous in the postero-lateral portion of the body. 



The fundamental vessels of the holostome t3'pe are the paired laterals and 

 the transverse vessel. Anastomoses and modifications have altered the 

 system appreciably, but not beyond the ability to recognize in them a common 

 type underlying the system, probably reducible to the Y-type of the distomes. 



The entire system contains many medium sized refractory granules v/hich 

 oscillate back and forth thru the vessels at every movement of the animal. 

 By unusual contraction of the worm, the granules are forced into the bladder 

 and out thru the excretory pore. 



C. Distomata. The distome cercaria has a simple type of excretory 

 system, consisting of a posterior median bladder with two lateral longitudinal 

 vessels in the body of the cercaria and a median longitudinal vessel in the tail. 



1. The xiphidiocercariae. The excretory system of the stylet cercariae 

 consists of a bladder, usually muscular, and a pair of lateral longitudinal 

 vessels which arise from lateral cornua of the bladder. The primitive lateral 

 system consists of three tubules, which emerge from the common lateral soon 

 after the latter leaves the bladder. One of these tubules proceeds posteriad, 

 while the other two run forward as inner and outer tubules. The single 

 median tail vessel ends blindly near the posterior end of the tail; it sometimes 

 receives tributaries, but this is not always the case. 



The bladder is a median posterior structure opening to the exterior thru 

 the dorsal pore. It is usually muscular, unpaired. It varies greatly as to 

 size and shape. It may be pyriform (Fig. 90), obovate (Fig. 93), crenate 

 (Figs. 55, 81), truncate or obtruncate (Figs. 60, 100), falciform (Looss, 1896, 

 Fig. 146), or lunar (Looss, 1896, Fig. 179). 



The physiological and morphological bladders are not always the same. 

 The former may encroach on the lateral tubules in order to increase its capa- 

 city. This may consist of mere enlargements of the cornua without any change 

 in structure (C. glandulosa, Fig. 60; C. micropharynx, Fig. 90). On the other 



