212 DISCOVERY REPORTS 



BLOOD SYSTEM 



THE PERICARDIUM AND ASSOCIATED MUSCULATURE 



The pericardial space, as in Doloria, consists of two distinct parts — the anterior or 

 main pericardium in which lies the heart and the posterior pericardium extending into 

 the trunk region. 



I have described the attachment of the anterior pericardial floor to the body wall 

 in Doloria as being, on each side, along a line like a distorted M. This arrangement is 

 due to the presence of three sets of muscles whose attachments occupy the bays, two 

 below and one above, formed by the M. 



In Gigantocypris the first bay, with its associated muscles, is present and in a similar 

 position. Of the muscles, however, while the aortic muscle and the nauplius eye muscle 

 are well developed, there appears to be no trace of the anterior eye muscle which I 

 described but did not figure (p. 451) for Doloria. This is not surprising considering 

 the vestigial nature of the paired eyes. 



The middle or upper bay has disappeared and is replaced by a long, drawn-out tube 

 of pericardial floor (Fig. 7 b). This has been brought about by the forward and outward 

 migration of the attachment of the pericardial compressor muscle which the bay accom- 

 modates. The migration has gone to such an extent that the bay has been drawn out 

 into a long V, until finally the two arms of the V have coalesced thus nipping off a small 

 island around the muscle attachment. 



The third or posterior bay has disappeared. In Doloria it exists to accommodate 

 the massive postero-dorsal eye muscle. As I have previously pointed out (p. 207), this 

 muscle in Gigantocypris is relatively slender and functions as a levator muscle for the 

 anterior sclerite system. Its point of attachment has migrated outwards so that it no 

 longer impinges on the pericardial floor attachment and the latter has straightened out. 



A comparison between the two anterior pericardia may be made by comparing Fig. 7 

 with Fig. \b of Doloria (Cannon, 193 1, p. 450). 



The posterior pericardium is more extensive in Gigantocypris than in Doloria. As 

 in the latter, it is divisible into right and left halves, the pericardial floor being confluent 

 with the ectoderm down the middle line as far back as the caudal furca. 



On either side the posterior pericardium consists of two parts — a postero-dorsal 

 (morphologically dorsal) lacunar space and an antero-lateral tubular space, the afferent 

 canal leading from the ventral part of the body cavity into the anterior pericardium. 



The lacunar part of the pericardium extends just as far as the area covered by the 

 circular muscles already described (p. 196, Fig. 9). Throughout this area the pericardial 

 floor is a thin membrane attached to the inner surface of the dorsal longitudinal muscles 

 and to the circular muscles and through these to the ectoderm. It thus reaches as far 

 back as the caudal furca, and at the sides of the body to that indefinite zone where the 

 circular muscles spread out into fibrils extending on the one hand into the ectoderm 

 and on the other into the pericardial floor itself (Fig. 9). 



