ON THE CCELOM, GENITAL DUCTS, AND NEPHEIDIA. 483 



Entoprocta. 



This group also, from our present point of view, differs little 

 in its organisation from the Planarians. 



Apair of nephridia — short tubes, generally with an intracellu- 

 lar lumen and ending in a flame-cell or a group of cells with 

 cilia, — open by a median pore in front of the genital pore (fig. 4) 

 (Hatschek, 47; Harmer, 46a; Ehlers, 29; Davenport, 26). 

 Possibly in some forms they open internally (Joliet, 61), and in 

 Urnatella they may be connected with a system of branching 

 tubules ending in flame-cells (Davonport, 26). The origin of 

 the nephridia is doubtful; Hatschek traced them in the embryo 

 to cells which he believed to be mesoblastic (47). 



Embedded in parenchymatous tissue are a pair of genital 

 follicles (fig. 4) . From each a typical peritoneal funnel leads to 

 a median pore (Ehlers, 29). 



MOLLUSCA. 



We now have to deal with a group of animals in which, as I 

 shall endeavour to show, the embryo is provided with a pair of 

 true nephridia; later, when the two ccelomic sacs belonging to 

 the unsegmented trunk have acquired a considerable size, 

 the peritoneal funnels formed from their walls take on the 

 excretory function, whilst the nephridia degenerate i. 



True nephridia have been described in all the groups of 

 Mollusca except the Isopleura and the Cephalopoda, and have 

 recently been the subject of a special study from Dr. R. von 

 Erlanger (34, 35), They consist of short tubules formed, as a 

 rule, of one or of a small number of cells, pierced by a canal 

 which communicates with the exterior by a pore behind the 

 velum (fig. 20). The inner end of the nephridium is provided 

 with a flame-like bunch of cilia, or with a flagellum. An 

 internal opening into the blood-space or head-cavity has been 

 observed in some forms, such as Lymnseus, Helix (de Meuron, 

 79; Jourdain, 62; Fol, 38; Sarasin, 92; Wolfson, 111; v. 

 Erlanger, 35, &c.). Teredo (Hatschek, 50), and perhaps in 



1 This view evidently obviates the difficulty some have felt as to the presence 

 of two pairs of so-called nephridia in an animal composed of one segment. 



