446 DISCOVERY REPORTS 



The main mass of the endosternite (Fig. 5), consisting of a transverse plate in which 

 are embedded the upper ends of the anterior hypostomal apodemes, curves upwards 

 at the sides and merges into the two halves of the adductor muscle. As a result of this 

 dorsal concavity of the adductor system, contraction of the muscle must lift the body 

 into the shell as the valves close. 



Laterally, the endosternite is prolonged forwards on either side of the oesophagus 

 (Fig. 5) as a thin, elongated triangular plate, the apex of which serves for the attachment 

 of the posterior eye muscles. The latter run forwards and attach to the sclerites c im- 

 mediately behind the eye stalks. 



The two anterior prolongations I have called the endosternite fork. The flat inner 

 ends of the antenno-labral apodemes are embedded in the proximal parts of the prongs 

 of the fork. 



Spanning the gap between the tips of the fork is the aortic tendon (Fig. 5). This is 

 an extremely thin tendinous sheet, in the median posterior part of which is embedded 

 the inner end of the frontal apodeme. 



From the level of the anterior hypostomal apodemes there occurs, on either side, 

 a thin strut connecting the adductor muscle tendon with the upper part of the stalk 

 of the antenno-labral apodeme (Fig. 5). 



Posteriorly there arise two very thin tendinous strands, which slope rapidly towards 

 the middle Hne and terminate in a pair of median plates (Fig. 5), one above the other, 

 which serve for the attachment of muscles running to the vibratory plate of the maxilla. 

 The lower plate also serves muscles from the maxillule. 



DORSAL BODY WALL 



I have already described the dorsal body wall between the heart and caudal furca 

 as a soft flexible dome. It consists of a chequer-work of muscles, formed of dorsal 

 longitudinal muscles and a series of transverse muscles — or, more accurately, circular 

 muscles — lying close inside the ectoderm and connected through it direct to the cuticle. 

 The latter are figured but not described by Miiller (1894, PI. 35, fig. 5). 



The dorsal longitudinal muscles consist on either side of about eight thin strands — 

 the number varies — running side by side from the caudal furca to the sides of the heart. 

 The arrangement and anterior attachments will be described later in connection with 

 the blood system. Throughout their length they are attached at intervals by short thin 

 tendinous strands, certainly to the ectoderm and probably through the latter to the 

 cuticle. 



From the mid-dorsal line numerous circular muscle bands extend round the sides 

 of the body (Figs. 2, 4). In extending ventrally they converge and fuse with the ecto- 

 derm, just below the most lateral of the dorsal longitudinal muscles. The ventral 

 attachment of the more anterior bands merge into an extremely thin tendinous sheet 

 connected with the e sclerite. The muscles lie just inside the ectoderm and are attached 

 closely to it by tendinous strands at very short intervals. 



