IV 



VERMESNEPHRIDIA 



239 



is their morphological significance. A certain number of the anterior and of the 

 posterior segments of the adult Hirudo are without nephridia. This fact favours 

 the conjecture that the larval nephridia of the Hirudinca are the provisional nephridia 

 of the anterior trunk segments. Possibly 

 the foremost pair of larval nephridia of the 

 Hirudinea represent the embryonic head 

 nephridia (head kidneys) of other Annulata. 



Concerning the structure of the pro- 

 visional trunk nephridia, the following 

 may be said. In the Capitellidce and 

 Oligochceta they show in general the same 

 structure as the permanent trunk nephridia. 

 In Xereis they are distinguished from the 

 permanent nephridia by the want of an 

 inner aperture opening into the body 

 cavity, i.e. of a funnel ; both by this fact 

 and the fact that the nephridial canal is 

 intracellular they recall the larval head 

 nephridia of many Annulata. The larval 

 nephridia of the Hirudinca have neither 

 inner nor outer aperture. 



C. The permanent nephridia. In 

 every Annulate nephridium, if we for the 

 time ignore the numerous complications 

 and modifications presented by the differ- 

 ent divisions, the following three portions 

 may be distinguished : (1) an inner 'cili- 

 ated aperture opening into the body cavity 

 or into a blood sinus ; this from its shape is 

 often called the funnel ; (2) a canal con- 

 nected with the above, which is generally 

 ciliated, and often has glandular walls ; 

 and (3) a terminal portion opening exter- 

 nally. The central part or nephridial canal 



is intracellular in the Hirudinea and FlG . 159 ._ Diagram of a very y0 ung sped- 

 Oligochceta, and generally much coiled men of Nereis cultrifera, after Edward Meyer, 

 (looped canal) (Fig. 160). In the Polychceta g, Brain ; au, eyes ; In, larval trunk nephridia ; 

 it is usually intercellular (lined with a P h > pharynx with jaws ; di, dissepiments ; d, 



many-celled epithelium) and not coiled in iniestin 1 e ; <> ventral ^> ed ' ^ ld : gu * ; ^ 

 J parapodia with cirri and setae. On the head are 



a complicated manner. The portion of the tentacles and sensory cirri, 

 the Annulate nephridium which projects 



into the body cavity is outwardly covered by a continuation of the peritoneal endo- 

 thelium., 



In the Hirudinea the permanent nephridia are wanting in a number of the 

 anterior and posterior segments. In the rest of the body they are found in strictly 

 segmental arrangement, one pair in each segment. The position of the funnels 

 in the body varies 'very much ; they lie either in the ventral blood sinus 

 (Clepsine\ or in those sinuses in which the testes lie (Hirudo, Aulostoma), or in 

 other blood sinuses of the body. The nephridial canal has many windings and 

 loops which lie close together, the finer details of which it is extremely diffi- 

 cult to make out. Finally, it opens externally either directly without terminal 

 swelling (Clepsine), or it opens into a vesicle lined with epithelium (ciliated in H. 



