MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 413 



first made known by Mitsuknri (No. 13). Instead of filaments, each gill is made up 

 of a number of flat plates, placed one against anotlier, the two gills on each side 

 being supported by a common muscular membrane, which is attached in the usual 

 position to the sides of the visceral mass. The palps in Nneula and Yoldia are very 

 large and extend back for some distance upon the visceral mass. The gills, pos- 

 terior to them, extend backward to the posterior end of the body. Fig. 92 rej)resents 

 half of two gills of Yoldia. The cut surface at the right of the figure exposes not 

 one continuous plate, but two plates, one on each side of the supporting membrane j«. 

 On the ventral side of the gill, opposite the supporting membrane, is a groove, not 

 well shown in the figure (^r), separating the plates of either side. Other evidence will 

 be given to show that these plates are really sei^arate, and one plate does not extend 

 entirely across the gill. 



This organ in Solenomya is very similar to the one described. Here (Fig. 91) the 

 gill on either side is attached to the visceral mass by a short supporting membrane 

 in such a way as to almost completely enveloj) the posterior part of the body. The 

 outer plates of the gill now extend uj)ward on the side of the body instead of hanging- 

 down below the point of the attachment of the supporting membrane, as in Yoldia. 

 The plates of the upper row differ in shape from those of the lower, being longer 

 and narrower. 



There is yet another conditi(ui of the respiratory organs of lamellibranchs, first 

 described by Prof. Dall, of the Smithsonian Institution (No. 4). In Cuspidaria and 

 Poromya, probably very far removed from primitive forms, the gill as such seems to 

 have disappeared. On either side of the body, extending from the walls of the visceral 

 mass out to the mouth, is a thick, horizontal, muscular membrane. It extends the 

 entire length of the animal, from close behind the anterior adductor back to the sii)ho- 

 nal seijtum, and separates the mantle chamber on either side of the body into an upper 

 and a lower chamber. Through this membrane open a number of orifices of various 

 arrangement, which allow a passage of water ujiward fi-om the lower into the upj)er 

 chamber. This latter corresponds to the usual epibranchial chamber, water obtained 

 from the chamber below being discharged through the anal siphon. The upper cham- 

 ber is stated by Dall (No. 5) to be used as a marsupium. 



There seems to be a question as to whether or not the gill has disappeared in 

 those forms and whether the muscular membrane is homologous with the gills or a 

 morphologically different organ. Dall (No. 4) some years ago expressed the view that 

 this membrane was not morphologically a gill, but that it was a great extension ante- 

 riorly of the muscular siphonal septum found greatly developed in other forms. A 

 gradual transition, in which the true gills become smaller while the siphonal septum 

 increases in area and extent, is traced through the forms Lyonsia, LyonsieUa., and Ver- 

 ticordia, all these forms possessing true gills. With the loss of the gills the septum 

 takes upon itself their function of respiration, and the progress of its specialization for 

 this purpose after the gills have disappeared is illustrated in the series of forms Myonera, 

 Cuspidaria, Ctenoconcha, and Poromya. There are possibly cases in which the mus- 

 cular septum is made up of structures diverse in their origin, the anterior part being 

 from the gills and the posterior from the siphonal septum. 



More recently Pelseneer homologized this septum with the gills (No. 17). The 

 reasons given for this view are that, while it is connected with the siphonal septum, 



