504 TRANSACTIONS OF THE WAGNER FREE 



onia, some Cardiums), are found the cases where no siphon or septum has been 

 developed ; the less active (Avicula, Pinna, Tridacna, Astarte) have at least a 

 septum of some sort, while the most perfected among the modern types (7W- 

 litia, Mactra, Venus, Mya, Yoldia) have a pair of well-developed siphons, fre- 

 quently supplemented by a septum. 



These siphons, being a local modification of the mantle margin, receive 

 their musculation from the same source. In general the muscles have spread 

 inward, pan passu, with the increase in length of the organ to be retracted, and 

 their insertion on the valve leaves scars in the shape of a sinus, which is an 

 important aid to classification of the minor groups. It has been sometimes 

 assumed that the absence of this sinus was evidence of the asiphonate character 

 of the species, but the example of Litcina, which has no sinus, and recent re- 

 searches in several other groups show that this is not necessarily true. In Cus- 

 pidaria with long siphons there is no pallial sinus, the retraction of the siphons 

 is accomplished by the contraction of the muscular septum. In Demiatoiitra, 

 with short siphons, the transfer of the muscular retractors from the mantle to 

 the septum can be seen in progress. There is no rule without more or less 

 marked exceptions, consequently the use of this feature in classification must 

 be confined to minor groups. 



The Gills Or Branchia. Of late, among morphologists recognixing 

 the partial character of the muscles and siphons as foundations for classifica- 

 tion, there has been a tendency to fall back on the characters afforded by the 

 breathing organs. Embryology shows that the gills take origin as a single 

 ridge on each side of the body, from which single filaments are put forth. 

 Diinya illustrates, among recent mollusks, this type of gill, but as in Dimya the 

 filaments are confined to one side of the ridge forming the base and are of the 

 ordinary filibranchiate type, except in wanting the reflection found in most fili- 

 branchs, it may be surmised that here we have merely a case of reversion to- 

 ward the ancestral type through degeneration of a gill which at one time bore 

 two rows of filaments. 



In a general way the ctenidium of a recent Pelecypod is composed of a 

 stem carrying a nerve and blood-vessel (sometimes both artery and vein), from 

 which on each side leaflets or slender filaments are given out laterally. In the 

 more archaic types (Nucula, Yoldia, Solemya) these leaflets are plate-like, not 

 organically united except by the stem, though in some cases obtaining some 

 solidarity, as a mass, by the interlocking of very large cilia, distributed in bands 

 or patches on the opposed surfaces of individual plates. 



In Eiicinxr the stem supports laterally elongated trilobate plates, the outer 

 limb united to the mantle distally, and the inner to each other behind the foot ; 

 the outer lobe corresponds to the reflected portion of its limb in other types of 

 gill. Near the stem a few organic connectives maybe found between opposed 

 plates, and the dorsal edges are knit by longitudinal muscular fibres, sparsely 

 disposed, but the main body of the plates are connected only by bands of inter- 



