90' PROCEEDINGS OF THE MALACOI.OGICAL SOCIETY. 



and to the body-wall, are those of a typical Eulamellibrancli. The 

 same is true of their more intimate structure, for an examination of 

 sections under the microscope reveals the fact that the two halves 

 of each gill lamella are connected together by vascular interlamellar 

 junctions, and that the gill filaments themselves are further similarly 

 united (Fig. II, i.l-j. and i.f-j.^. In the individual filaments we fijid 

 a supporting tissue, and firmer supporting rods (Fig. Ill, s.r.) are 

 present. The whole structure recalls in its most minute details that 

 of the gills of Anodonta or Unio. 



The heart consists of a muscular ventricle (v.) and two thin- walled 

 auricles {au.), the former giving origin to an anterior and a posterior 

 aorta, the whole being enclosed in a fairly spacious pericardium (pc). 

 A mass of brown tissue forms the side walls of the pericardium 

 anteriorly ; this evidently represents Keber's organ. 



The most striking feature seen in connection with the heart is the 

 fact that the ventricle is not perforated by the rectum, but is situated 

 some little distance below the latter, being separated from it by 

 a portion of the genital gland. The pericardium, moreover, does not 

 surround the rectum as in Anodonta, its roof being formed by the 

 thick mass of the genital gland, within which the rectum lies 

 embedded. 



This non-perforation of the ventricle by the rectum is a very striking 

 and peculiar feature, and one that is only met with elsewhere in such 

 monomyarian forms as Meleagrina, Ostrea, Anomia, and Pecten ; while 

 among the dimyarians it is found in Nucula, Area, and Teredo. 



The fact that MuUeria is a sedentary monomyarian, and that it, at 

 the first rough glance, recalls an oyster, suggests perhaps that the 

 non-perforation of the heart by the rectum has some phylogenetic 

 significance which further connects these two forms. A careful con- 

 sideration, however, of the other anatomical features in the two 

 genera, such as the structure of the gills, the relations of the kidney 

 and genital ducts, shows that this cannot be the case. In this con- 

 nection it is interesting to read the account given by Lang,^ who, not 

 knowing of the condition in MuUeria, explains the separation of the 

 heart from the rectum as due to the increasing distance between the 

 base of the gills and the original position of the heart, brought about 

 by the shifting forwards of the enlarging posterior adductor muscle. 

 The truth of this interpretation is rendered evident by an examination 

 of such heteromyarians as Pinna, Avicula, and Perna, in which the 

 consecutive stages in the separation of the heart from the rectum, 

 leading up to the complete displacement found in Ostrea, may be seen. 



One may, I believe, justly conclude that the same process has taken 

 place in MuUeria, and thus regard the independently acquired mono- 

 myarian condition of this genus as responsible for the separation of the 

 heart from the rectum. That the ventricle is not so distantly removed, 

 nor the whole heart so much elongated, as in Ostrea, may be explained 

 by the fact that in MuUeria the single adductor muscle is smaller, and 

 has not migrated so far forward as in Ostrea. 



1 Laug, Text-Book of Comparative Anatomy, English edit., part ii, p. 206. 



