JOU8SEAUMIA. 251 



over the posterior adductor muscle to end in the anus. The rectum traverses the 

 pericardium, and is wrapped round by the ventricle. 



THE CIRCULATORY SYSTEM is of the typical lamellibranchiate character, and 

 requires no special description. The ventricle, as has been mentioned above, is 

 traversed by the rectum. The auricles are excessively thin and can only be dis- 

 tinguished with difficulty in sections. Owing to the minute size of the animal the 

 relations of the principal blood sinuses could not be determined with certainty, but I 

 was able to distinguish a large ventral sinus above the muscular band formed by 'the 

 anterior and posterior retractor muscles of the foot, and there are the usual afferent 

 and efferent branchial sinuses at the bases of the gills. 



THE GILLS, as may be seen by an inspection of fig. 1, are of a very simple type. 

 The outer demibranch is wanting, a feature which Jousseaumia shares with the 

 Lucinidse, Corbis, Scioberetia and the Teredinidae. The direct lamella of the inner 

 demibranch is always well developed, and may be described as consisting of about 

 18 filaments, united at regular intervals by three, or in large specimens by four, rows 

 of non-vascular interfilamentar junctions. The reflected lamella is present in many 

 adult individuals, but is either absent or very feebly developed in others, and it is 

 always absent in young and immature specimens. When present, it is confined to 

 the anterior region of the demibranch, and the upper edge of the reflected lamella is 

 fused to the body wall along the line of junction of the foot and the visceral mass. 

 Posterior to the foot, where the reflected lamella is absent, the lower edge of the 

 direct lamella of one side is, in all but very young individuals, fused with the lower 

 edge of the corresponding lamella of the other side. If the reflected lamella is 

 absent in the region of the foot, its place is taken by a continuous sheet of mem- 

 branous tissue, which is attached to the sides of the upper part of the foot. Below 

 and behind the posterior adductor muscle the upper edges of the direct lamellae are 

 connected with the mantle (figs. 18 and 19), and the result of this arrangement is 

 that the gills divide the pallial cavity into an inter-lamellar or supra-branchial 

 chamber, opening behind by the anal pallia! aperture, and a large infra-branchial 

 chamber. 



Though I have spoken of filaments, the gills are not developed as separate 

 filamentar outgrowths which subsequently form the above described unions with 

 one another and the body wall and mantle, but by the fenestration of a pair of 

 lateral folds of the body wall, as has been described by other authors for Cyclas 

 (STEPANOFF, 14), Teredo (HATSCHEK, 7), and Scioberetia (BERNARD, 2). Although I 

 have not been successful in finding the earliest stages of gill development, I have a 

 complete series of post-larval stages showing that the fenestration proceeds from 

 before backwards, and that new fenestrse are added at the posterior ends of the 

 two gill membranes until the adult stage is reached. Fig. 27 represents a young 

 J. heterocyathi in which there are five fully formed fenestrations and the commence- 

 ment of a sixth posteriorly. Fig. 28 is a drawing of a somewhat older individual 



2 K 2 



