GIGANTOCYPRIS MULLERI 195 



excessive amount. In Doloria I have preparations in which there is certainly more fluid 

 in the trunk than in any of my Gigantocypris preparations. On the other hand, I have 

 sections in which the whole trunk region appears to be full of gut parenchyma. I 

 remarked on the variation of this tissue in Doloria (193 1, p. 462) and suggested that it 

 was probably correlated with food storage. I think now that this variation is more 

 apparent than real. What varies is not so much the parenchyma as the blood. If there 

 is a lot of blood then this gives the appearance of a relatively small amount of gut 

 parenchyma and of a distended body. 



A large amount of blood in the trunk will occur when the body is drawn well into the 

 shell. If, now, the circular and dorsal longitudinal muscles contract, then since they 

 together form a dome-shaped cover to the trunk region, they must press on the body 

 fluid. Since the blood is incompressible it must, as a result of this pressure, move out 

 of the trunk into the anterior part of the body. This will push down the mouth region 

 and associated mouthparts towards the ventral slit through which the mandibular 

 palps can now be thrust. Now the mouthparts, as I describe in detail later, are rigidly 

 connected to the adductor tendon. When the dorsal body wall contracts this whole 

 ventral mass moves downwards, and, as a result, the adductor muscle and its median 

 tendon must become bowed. Attached to the median tendon there are two muscles, the 

 dorso-ventral body retractors (Plate XXXIX, fig. 2). These commence near the middle 

 point, each as a thin tendon which then runs as a double muscle to attach to the outer 

 layer of the valves (Fig. 8) just posterior to the attachment of body to shell. The muscles 

 sometimes bifurcate so as to give a quadruple attachment to the shell (Fig. 5, muscle 10, 

 and Fig. ja). These muscles are the antagonistic muscles to the dorsal body wall. When 

 they contract they must tend to pull the ventral body mass upwards into the cavity of 

 the shell. The whole part of the body, ventral to the adductor muscle, can thus be looked 

 upon as a system the movement of which is hydrostatically controlled by the co-ordinated 

 activity of the muscular dorsal body wall together with the dorso-ventral body retractor 

 muscles. 



CONSTITUTION AND ORIGIN OF DORSAL BODY WALL 

 The constitution of the dorsal body wall has been described briefly for Doloria 

 (Cannon, 1931, p. 446). In Gigantocypris I have been able to make out its structure in 

 much greater detail. The outer layer consists of a thin ectoderm, consisting of an epi- 

 thelium of flattened polygonal cells containing nuclei which are so flat that they do not 

 protrude from the surface. Inside this, and attached at short intervals to the ectoderm, 

 are the circular muscles, commencing from near the mid-dorsal line and running on 

 either side to the mid-lateral part of the body. Inside this again, and attached both to 

 the circular muscles and through them to the ectoderm, are the dorsal longitudinal 

 muscles, running from the sclerite of the caudal furca to the frontal region, that is, 

 at right' angles to the circular muscles. Finally, attached to the dorsal longitudinal 

 muscles, and in their plane, is a thin sheet, the posterior pericardial floor. 



The ectoderm is covered on the outside by a thin layer of cuticle. It is difficult to 



