292 DEVELOPMENT OF ORGANS. 



Excretory organs. There are found in the embryo of Nephelis 

 and Hirudo certain remarkable provisional excretory organs the origin 

 and history of which is not yet fully made out. In Nephelis they 

 appear as one (according to Robin, No. 364), or (according to Blitschli, 

 No. 359) as two successive pairs of convoluted tubes on the dorsal 

 side of the embryo, which are stated by the latter author to develop 

 from the scattered mesoblast cells underneath the skin. At their 

 fullest development they extend, according to Robin, from close to the 

 head to near the ventral sucker. Each 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 researches to believe that they 

 open laterally. They contain a clear fluid. 



In Hirucfo, 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 com- 

 mences with an enlargement from which a convoluted tube is con- 

 tinued 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 labyrinthic network. 



The true segmental organs are found in a certain number of the 

 segments and are stated (Whitman) to develop from groups of meso- 

 blast cells. Their origin requires however further investigation. 



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

 described in Clepsiiie 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 of 

 the mesenteron is at first formed of the three vitelline spheres, 

 from the surface of which a true hypoblastic layer enclosing a central 

 yolk mass becomes differentiated, as already described. The mesen- 

 teric sack so formed is constricted by the growth of the mesoblastic 

 septa into a series of lobes, while the posterior part forms a narrow 

 and at first very short tube opening by the anus. 



The lobed region forms the sacculated stomach of the adult. The 

 sacculations of the stomach by their mode of origin necessarily corre- 



1 Morpholog'ischcs Jahrlitch, Vol. iv. p. 676. He further speaks of the tube as 

 "feinverzweigt u. uetzforrnig verastelt," but whether from his own observations is 

 not clear. 



