452 DISCOVERY REPORTS 



that is, the most median, I have already mentioned as being a continuation, through a 

 tendinous junction, of the pericardial compressor. The second runs forward to attach 

 directly to the carapace immediately behind the upper attachment of the nauplius eye 

 muscle. The third attaches close behind the second. 



Lateral to these three muscles is a group of five more, the fourth to the eighth. These 

 also attach anteriorly to the carapace valves, and in a line with the attachment of the 

 more median muscles. The more lateral muscles attach more posteriorly, that is, the 

 fourth attaches just behind the third, and the eighth has the most posterior attachment. 

 As they pass out of the anterior pericardium they lie close over the pericardial floor and, 

 in side view, appear to cross each other. They extend a short distance behind this level 

 and attach directly to the ectoderm (Fig. 2). From here they run parallel, and are 

 attached at intervals to the ectoderm and to the circular muscles. They curve ventrally 

 and end in a tendinous plate immediately inside the ectoderm, which extends forwards 

 to the lower limit of the e sclerite. 



The lateral attachment of the anterior pericardial floor, on entering the trunk region, 

 passes posteriorly just outside the eighth dorsal longitudinal muscle. The floor of the 

 posterior pericardium does not extend right across the body, but has a medial line of 

 attachment. This extends from the median ventral point of the hind wall of the anterior 

 pericardium, across the first dorsal longitudinal muscle and, after sending a small bay 

 down the second and third muscles, extends down the medial side of the fourth (Figs. 

 4 A, 5). The posterior pericardial floor thus lies close underneath the lateral group of 

 dorsal longitudinal muscles. It can be traced almost to their posterior limits where it 

 disappears. 



The middle line of the anterior pericardial floor is pulled down into the form of a V 

 by a powerful muscle, the pericardial dilator (Figs. 4 A, 6). The anterior arm of the 

 V is split along the middle portion of its length. The sides of the split have developed 

 continuous myofibrils and are fused with the walls of the heart. The split forms the 

 aortic valve by which the heart communicates with the aorta. The posterior arm of 

 the V consists of a well-marked band of continuous myofibrils — the median sub- 

 pericardial muscle. 



The apex of the V is rounded, and along the curved part is attached the pericardial 

 dilator. In side view (Figs. 2, 4 A) this appears as a fan-shaped muscle spreading out 

 to its dorsal attachment. It is, however, continuous posteriorly with the median sub- 

 pericardial muscle and anteriorly with the muscles in the lips of the aortic valve. In 

 passing ventrally, it spreads out laterally and attaches along a transverse line to the 

 aortic tendon. Actually, the shape of the muscle is that of a tetrahedron with curved 

 edges (Fig. 6). 



Passing upwards from the e sclerite, that is, just behind the attachment of the 

 adductor muscle, there is a thin tendinous sheet, which narrows down and becomes 

 muscular as it extends dorsally. It lies at first immediately inside the ectoderm and, 

 while still in this position, enters the posterior pericardium (Fig. 4). Here it runs 

 obliquely forwards across the eighth, seventh and sixth dorsal longitudinal muscles, 



