igoS.] EXCRETORY ORGANS OF METAZOA. 677 



22. EnTEROPNEUSTA. 



Xcphridia. — There is a left canal (or a right and left) con- 

 necting the coelom of the proboscis with the exterior, a pair of 

 similar canals in the collar region, and in Spengelia (Willey, 1899) 

 rudimentary pores along the whole trunk. Spengel (1893) con- 

 sidered them to take in water from without and to subserve loco- 

 motion ; Willey regarded them as having lost their former excretory 

 function, while Bateson (1884) showed by carmine injection that 

 the collar pores are excretory. 



An ectoblastic origin of these structures was the result of the 

 study of Spengel and Morgan (1894). But DawydofiF (1907), 

 examining those of the proboscis in the process of regeneration, 

 found that they develop from a peritoneal evagination that connects 

 with an ectoblastic ingrowth, and from this concluded that they 

 are true nephridia — a view previously reached by Schimkewitsch 

 (1888). 



The genital ducts seem to bear no relation to nephridia, and the 

 larva (tornaria) lacks special excretory organs. 



Glomerulus. — A vascular structure connected with the peri- 

 cardium, considered the only excretory organ in the adult (Willey, 

 1899) ; I have not seen the original description and consequently am 

 unable to add further details. 



23. MOLLUSCA. 



Adult Nephridia. — I have not attempted to labor through the 

 compendious literature on the anatomy of these organs, but shall 

 simply give a brief summary drawn mainly from the excellent treat- 

 ment by Hescheler (1900). These are essentially similar and 

 homologous throughout the group, and consist typically of a pair 

 of sacs which communicate internally by open nephrostomes 

 (renopericardial apertures) with the coelom (pericardial cavity), 

 and externally by nephridiopores with the mantle cavity. They are 

 paired in all the groups except the Gasteropods, and among the latter 

 in most of the diotocardial prosobranchs ; among living forms there 

 is more than one pair only in Nautilus. They may be simple tubes, 

 or may be twisted or excessively ramose. Functionally they mav 



