DISCOPHORA. 353 



convoluted tubes on the dorsal side of the embryo, which are 

 stated by the latter author to develop from the scattered meso- 

 blast cells underneath the skin. At their fullest development 

 they extend, according to Robin, from close to the head to near 

 the ventral sucker. Kach of them is U-shaped, with the open 

 end forwards, each limb of the U being formed by two tubes 

 united in front. No external opening has been clearly made 

 out. Semper believed that the tubes were continuous with 

 the three posterior vitelline cells, but this has been shewn not 

 to be the case. Fiirbringer 1 is inclined from his own re- 

 searches to believe that they open laterally. They contain a 

 clear fluid. 



In Hirudo, Leuckart (No. 362) has described three similar 

 pairs of organs the structure of which he has fully elucidated. 

 They are situated in the posterior part of the body, and each of 

 them commences with an enlargement from which a convoluted 

 tube is continued for some distance backwards ; it then turns 

 forwards again and afterwards bends upon itself to open to the 

 exterior. The anterior part is broken up into a kind of laby- 

 rinthic network. 



The true segmental organs are found in a certain number 

 of the segments and are stated (Whitman) to develop from 

 groups of mesoblast cells. Their origin requires however further 

 investigation. 



A double row of colossal cells on each side of the body has been 

 described in Clepsine by Whitman as derived from the mesoblastic plates. 

 These cells (fig. 58 B), which he calls segment-cells, lie opposite the walls of 

 the septa. The inner row is stated to be connected with the segmental 

 organs. Their eventual history is unknown, but they are conjectured 

 by Whitman to be the mother cells of the testes. 



The alimentary tract. This is formed primitively of two 

 parts the epiblastic stomodaeum forming mouth, pharynx, 

 and oesophagus, and the hypoblastic mesenteron. The anus is 

 formed very late as a simple perforation immediately dorsal to 

 the posterior sucker. 



In Clepsine, where there is an epibolic gastrula, the rudiment 



1 Morpkologisckts Jahrkuth, Vol. iv. p. 676. He further speaks of the tube as 

 " feinverzweigt u. netzformig vera&telt," but whether from his own observations is 

 not clear. 



B. II. 23 



