2 i6 DISCOVERY REPORTS 



I consider function by controlling the bore of the main afferent blood channel leading 

 into the pericardium. On their contraction the canal becomes a wide open tube. On 

 relaxation the internal pressure produced by the dome of circular and longitudinal muscles 

 will press the floor of the canal against the ectoderm forming its roof and so close it. 

 The functions of these two sets of muscles are therefore what I ascribe solely to the 

 lateral subpericardial muscle in Doloria. The interaction of the muscles associated with 

 the pericardium during the complete heart beat, which I described for Doloria (Cannon, 

 193 1, p. 456 and Fig. 7, p. 460), is not upset by this change. If to the name "lateral 

 subpericardial muscles" is added "lateral series of dorsal longitudinal muscles" the 

 same argument applies. 



The lateral subpericardial muscle is attached directly as a thin powerful strand to 

 the mid lateral body wall, in that zone between the afferent canal and the isthmus of 

 attachment of body to shell (Fig. 9). From the point where it disappears as tendo-fibrils 

 in the body wall two muscles are attached, which I have previously numbered as 4 and 1 1 

 (Plate XLII, figs. 9, 10). Of these, no. 4 is used to move the upper part of the tripod 

 in the sclerite system and is of little significance in connexion with the muscles we are 

 discussing. No. 11, however, runs across the isthmus to attach at the base of the 

 a sclerite, at a point which, from my previous description, and as can be seen from 

 Fig. 5, is one of the most stable points in the whole body. The lateral subpericardial 

 muscle is thus connected firmly to the main skeletal system. 



At its lower end, immediately above its tendinous attachment, it is definitely a striated 

 muscle, but as it passes dorsally and enters the pericardial floor it spreads out and its 

 striations become less marked until when it enters the main pericardium its myofibrils 

 are continuous. In Doloria I described it as disappearing towards the region of the 

 hepatic valves. In Gigantocypris I have been able to follow its course accurately. As it 

 passes over from the afferent canal into the main pericardium it spreads out into several 

 branches. The most posterior branch extends almost directly inwards, disappearing 

 towards the middle line. The more anterior branches behave in a similar way but extend 

 proportionately more forwards. In fact all branches, except the most anterior which 

 is a thin strand, extend towards the middle line and so towards their fellows from the 

 opposite side. As a result of this arrangement the pair of muscles must form an arch, 

 the bases of which are firmly connected to the main skeleton. Contraction of the pair 

 of muscles together must therefore pull down the roof of the arch, or, in other words, 

 depress the floor of the main pericardium. Individually they will at the same time depress 

 the floor of the upper end of the afferent canal and so aid the lateral series of dorsal 

 longitudinal muscles in opening up this passage. 



The thin strand which forms the most anterior branch of this spreading muscle I have 

 traced forwards to the hepatic valve itself (Fig. 10). 



