436 BULLETIN OF THE 



ent. The lamella) of the Gastropod are destitute of the elaborate framework 

 of chitine found in the Pelecypod ctenidium. 



In referring to the insufficiency of characters offered by the specialization of 

 the gill structure for systematic purposes, it was not my intention to deny 

 these characteristics any value. They have a certain value in minor groups, 

 such as families, but, in my opinion, far less for higher groups than that of the 

 nervous system, the relations of the heart and intestine, the radula and the 

 shell, all of which are more fundamental in connection with the molluscan 

 organization. This is hardly the place to enter into a general disquisition on 

 the ctenidia, but it may not be amiss to observe that two chief factors in their 

 position, specialization and development, are the desirability of avoiding the 

 water fouled by effete matter discharged from the intestine, and the economic 

 use of tissue to obtain large aerating surface with the least expenditure of 

 material. These factors necessarily vary with many modifications of the 

 organism which are of little systematic importance, and consequently the 

 specializations of the gill subsequently produced are of no greater systematic 

 value than their inciting causes. The increase of aerating surface is attained 

 in a multiplicity of ways, many of which are most admirable and complicated; 

 but their significance is not great in the assignment of systematic position to 

 the animal which exhibits them. 



There can be little doubt that the original form of gill was a simple pinched-up 

 lamella. This, elongated, becomes a filament. Filaments united by suitable 

 tissue, trussed, propped, and stayed by a chitinous skeleton, result in the forms, 

 wonderful in number and complexity, which puzzle the student to describe, 

 and much more to classify them. 



But when it is a question as to deriving the greatest benefit from pure 

 water, if ret[uired (so to put it) to choose between the retention of the normal 

 .ctenidium on the one hand, and the development of new breathing organs 

 elsewhere with a possible obliteration of the original ctenidium on the other, 

 the organism will always give the functional processes great weight. In the 

 Docoglossa we have an instance of this. "Waiving the question whether the 

 blind deep-water Ahranchiata (^Le'petidcB, etc.) are degenerate or primitive 

 forms, it is evident that the Acmceidce are the most typical forms of the whole 

 group. They have a smooth muzzle with an elevated tactile margin, often 

 angulated at its lower corners; a normal ctenidium over the neck;, eyes; and 

 a rather primitive dentition. In the LejJetidce there are no eyes ; the tactile 

 margin of the muzzle has become prolonged into large tentacles ; there are no 

 specialized breathing organs of any sort; the radula is somewhat less primitive, 

 but still has rather generalized characteristics. 



In the forms where the effete products of intestine and kidney are discharged 

 over the neck into the same general chamber with the ctenidium, some dis- 

 advantage necessarily must result to the animal. In Scurria there are attempts 

 to remedy this by adding a more or less complete cordon of plain lamellsD 

 developed in the peripedal commissure between the mantle and the foot. 

 This begins as far as possible from the excretory organ?, and is developed last 



