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 another, tbe 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 Nucula 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 represents 
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 m. 
On the ventral side of the gill, opposite the supporting membrane, is a groove, not 
well shown in the figure (gr), separating the plates of either side. Other evidence will 
be given to show that these plates are really separate, 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 envelop the posterior part of the body. The 
outer plates of the gill now extend upward 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 condition of the respiratory organs of lamellibranchs, first 
described by Prof. Dali, 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 siplio- 
nal septum, 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 upward from the lower into the upper 
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 Dali (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. Dali (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, Lyonsiella , 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 My oner a, 
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, 
