The life history of Diplodiscus temporatus Stafford. 60Ö 



Excretory system (Water vascular system). 



In the living' cercaria the main trunks of the water vascular 

 system are the most prominent structures in the body. They appear 

 as two large tubes, about three eights as large as the intestinal 

 caeca, and are made up of large cells with highly refractive nuclei. 

 The excretory bladder is situated near the posterior end of the 

 body and is connected by a short canal with the excretory pore 

 which lies just anterior to the tail on the dorsal surface of the 

 body (Fig-. 7). The two main trunks of the water vascular system 

 extend out from the excretory bladder, one toward either side. At 

 the posterior end of the caecum of the corresponding- side the water 

 vascular trunk turns forward and extends along- the external side 

 of the caecum, and beyond this to near the region of the pharynx. 

 Throughout its length, but particularly in its posterior half, each 

 tube is highly convoluted. Some of the folds lie above and some 

 below the level of the caecum. At its anterior end the main water 

 vascular trunk breaks up into a number of branches, which are 

 distributed to all the tissues of the body. 



The nervous system. 



The brain, which consists of two lobes connected by a broad 

 commissure, is situated just posterior to the pharjnigeal pockets. It 

 is made up of a mass of fibres among which there are scattered 

 ganglion cells. The brain and the larger nerve trunks are sur- 

 rounded by a well defined layer of nuclei which Ziegler (1883) took 

 to be the ganglion cells (he mentions them only in connection with 

 the brain). These nuclei differ very markedly from the ganglionic 

 nuclei while, on the contrary, they differ hardly at all from the 

 nuclei throughout the parenchyma. My own observations in regard 

 to the nature of these nuclei are in agreement with those of 

 Schwarze (1886) and Tennent (1905) who hold that these nuclei 

 constitute a rudimentary sheath for the nervous system. The early 

 stages in the development of the cercaria show clearly the paren- 

 chymatous nature of these nuclei. 



From the brain there are given off anteriorly four nerve trunks. 

 These soon divide, the branches being distributed to the organs at 

 the anterior end of the body, the anterior sucking disc and the 

 pharyngeal pockets being particularly well supplied with nerves. 

 Posteriorly six longitudinal trunks are given off from the brain, 



Zool. Jahvb. XXVIII. Abt. f. Anat. ■iO 



