366 Ralph S. Lillie 



exliibit occasionai interruptions in its continiiity whicb raight admit 

 of the passage of minute bodies such as the early oögonia info the 

 interior of the vesicle. Leueocytes appear always to be present 

 tlierc to a greater or less degree. Definite openings however, 

 placing its interior in free communication with the body-cavity, do 

 not exist. The cavity in fact appears simply to represent an enlarged 

 lymph-space. It is lined by a tbin layer of connective tissue, 

 containing minute blood-vessels and occasionai nuclei, and directly 

 eontiniious with the membranous portion of the wall of the vesicle. 



The remainder of the ventral lip below the tubulär margin is, 

 like ali the non-marginal portion of the nephrostomial wall, composed 

 of (1) an internal layer of cubical ciliated epithelium, lining the 

 interior of the nephrostome; and (2) an external connective tissue 

 layer bearing blood-vessels and continuous with the peritoneum 

 covering the outer surface of the eutire organ. 



The manner in which the ventral lip acquires its characteristic 

 structure is at present somevvhat uncertain. The vesicle and marginai 

 tube make their appearance at a relatively late period of develop- 

 ment and are not present in larvae of 10 — 15 mm., the largest I 

 bave so far rearcd. The probable manner of their formation can, 

 however, be inferred from a comparisou of the adult and larvai 

 nephrostomes. In larvae of the above dimensions the nephrostome 

 is formed of a single layer of cubical ciliated cells covered extern- 

 ally by a thin peritoneal layer (Piate 25, Figs. 43, 44, 48 etc.) and 

 possesses an entire margin, along whose dorsal portion runs the 

 nephrostomial vessel (Figs. 42 — 49). The ventral lip is shorter than 

 the dorsal lip , but in other respects possesses a similar structure 

 (Fig. 47). The formation of a tubulär margiu had its origin, in ali 

 probability, primarily in an increase in the extent of the ciliated 

 surface. In the adult organ the originai border of the nephrostome 

 is undoubtedly represented by the line of Separation between the 

 epithelial and membranous portions of the tubulär margin (Piate 22, 

 Fig. 6). The membranous portion apparently represents the originai 

 peritoneal layer, which is bere no longer (as originally) in immediate 

 contact with the outer wall of the nephrostome, but bas become 

 separated from the lattcr by the formation of a marginai lymph 

 space of vcry definite form. The manner in which the present 

 couditions arose was thus very probably somewhat as follows: — 

 The epithelium of the ventral lip undcrwent a process of extension 

 in an outward direction: if the area of the epithelium increased 



