BLOOD SYSTEM 451 



not, however, give any figures, and this is undoubtedly incorrect. He missed com- 

 pletely the pericardial floor and, without this, the functioning of the heart cannot be 

 clearly understood. 



The pericardium consists strictly of two parts, the anterior pericardium in which 

 lies the heart (Fig. 4 A) and the posterior pericardial space which extends into the 

 trunk (Fig. 2). 



The anterior pericardium is roofed and walled laterally by the carapace. The front 

 wall is formed by the body wall above the nauplius eye, and the hind wall by a soft 

 flexible portion of the body wall which bulges in between the trunk ventrally and the 

 carapace dorsally. The pericardial floor can be considered as a membrane slung from 

 a median point just above the nauplius eye and the median point which marks the 

 anterior limit of the soft flexible dome of body wall covering the trunk. Laterally it is 

 attached partly to the body wall and partly directly to the valves of the carapace. Its 

 line of attachment has been twisted into a distorted M by certain muscles, whose inser- 

 tions lie in bays formed by its lateral margins. This is illustrated in Figs. 4 A, B. The 

 former represents the right half of a specimen of Doloria and the latter is a drawing 

 of the left half of the pericardial floor and heart which has been removed to obtain 

 Fig. 4 A. These figures are based on various series of sections. It is quite impossible 

 to see the actual floor in dissection as its extreme thinness renders it invisible. 



The first bay is occupied by the common dorsal attachment of three separate muscles. 

 The first of these is a muscle which nms from the median ectoderm just below the 

 nauplius eye and extends up on either side to attach to the carapace valves just lateral 

 to the hinge and immediately in front of the most anterior of the dorsal longitudinal 

 muscles which attach directly to the carapace. I have called it the nauplius eye muscle 

 (Figs. 2, 4 A, 6). The second is the aortic muscle which I shall describe later and the 

 third, a muscle which I have not figured, running to the paired eye, the anterior eye 

 muscle. 



The pericardial floor, after curving upwards to accommodate this attachment, re- 

 curves sharply downwards to pass underneath the attachment of a muscle running in 

 the pericardium itself. This runs from a point just above the eye stalk to a point slightly 

 lateral to the mid-ventral region of the hinder wall of the anterior pericardium (Figs. 2, 

 6). Here it is joined to its fellow by a tendinous strand, and continues after a tendinous 

 junction as the most median of the dorsal longitudinal muscles. This muscle I have 

 called the pericardial compressor. 



The attachment of the pericardial floor now passes postero-dorsally to curve round 

 the posterior attachment of a big muscle (not figured), the postero-dorsal eye muscle. 

 Passing forwards along the posterior face of this muscle it turns sharply backwards and 

 enters the trunk region as the lateral margin of the posterior pericardium. 



In order to explain the posterior pericardium I must describe in more detail the 

 dorsal longitudinal muscles. Passing from the hinder wall of the anterior pericardium 

 and extending posteriorly to the caudal furca are three band-like muscles which may 

 bifurcate and again join up before reaching their hinder limit (Fig. 2). The first of these. 



