2i4 DISCOVERY REPORTS 



Anterior to this the pericardial floor sinks into the body and then out again, thus 

 forming a gutter (Fig. yb) the anterior margin of which attaches to the lateral body wall 

 along a line indicated in Fig. 9. Between this line and the posterior margin of the isthmus 

 of attachment of body to shell there is no pericardial space. This gutter that I have 

 just described is the afferent channel leading into the main pericardium (Plate XLII) 

 and corresponds to the branchio-cardiac canals of the Crayfish. It differs from the latter 

 in leading from the general body cavity and not from gills, and corresponds to the similar 

 channels described by Lowe (1935, p. 579) in Calanus. 



The dorsal longitudinal muscles are closely similar to those of Doloria, but in the 

 latter I think that I described some of the attachments incorrectly owing to the difficulty 

 in tracing the actual extent of the pericardial floor. In Gigantocypris this is never in 

 doubt, and I feel certain that the attachments I am now going to describe would apply 

 to any ordinary Cypridinid (Fig. 9). 



Enumerating the dorsal longitudinal muscles from the median line outwards, the 

 first muscle extends from the anterior attachment I have described in detailing the 

 outline of the anterior pericardium (p. 212). It runs obliquely inwards and backwards 

 and attaches to the ectoderm just lateral to the middle line at the hinder margin of the 

 anterior pericardium. The pair of muscles then run parallel to each other backwards to 

 the caudal furca, being closely attached to the ectoderm throughout their length. The 

 anterior part of these muscles will form the pericardial compressor muscles — the 

 posterior part, the first dorsal longitudinal muscle. 



The second, third, fourth and fifth dorsal longitudinal muscles attach anteriorly in 

 a group direct to the outer shell of the valves which forms the lateral wall of the anterior 

 pericardium. The second and third muscles run backwards to the caudal furca, as does 

 the first, but none of these three run over this length as continuous muscle. They all 

 gradually dwindle posteriorly and become a tendinous strand attaching near the caudal 

 furca, but in each case as they dwindle they are replaced by a tendinous attachment 

 which, on the contrary, increases in size and becomes muscular as it passes posteriorly 

 (Fig. 9). Because of this, sections of the body wall give the appearance of thin muscles 

 of uniform thickness running continuously over this stretch. It is only carefully stained 

 preparations of the whole body wall which show the oblique discontinuity in the muscles. 



The fourth dorsal longitudinal muscle is short and attaches to the ectoderm after 

 having traversed only about one-quarter of the length of the dorsal dome (Fig. 9 and 

 Plate XLII, figs. 1-4). It may be that it is a portion of the fifth muscle which has split 

 off, as its anterior attachment is the same as that of the fifth. 



The fifth dorsal longitudinal muscle runs backwards and downwards in an arc, just 

 projecting into the lacunar region of the posterior pericardium, but then emerging 

 again ventrally to disappear as a mass of tendo-fibrils in the actual pericardial floor at the 

 lower end of the afferent canal (Plate XLII, figs. 2-1 1). 



In this same region are attached the ventral tendinous portion of the sixth, seventh 

 and eighth dorsal longitudinal muscles (Plate XLII, fig. 8). These run upwards in the 

 afferent canal, curving forwards and inwards to attach to the outer shell forming the wall 



