zoo DISCOVERY REPORTS 



This, I maintain, was produced by an internal body pressure tending to inflate and so 

 put in a state of tension this softening dorsal body wall. A tense body wall is rigid and 

 hence would provide points of attachment for the dorsal longitudinal muscle. Now 

 tissues that are stretched often develop fibres, and I have shown in a series of forms all 

 gradations between the appearance of simple fibres leading to typical striped muscle. 

 This is what I consider has taken place in the Ostracodan body wall. The body pressure 

 led to a state of tension in the ectoderm and this led to the development of fibres which 

 eventually evolved into striated muscle. The way in which I consider that these ectoderm 

 cells, in which the myofibrils developed, sank in and became internal muscles is illus- 

 trated at one point on the preparation of isolated ectoderm to which I have already 

 referred (p. 196). Here two ectoderm cells, which are not adjacent, are seen to be 

 connected together internally. That is, on the inner face of the ectoderm there is a bridge 

 spanning a gap about two or three cells wide and joining two ordinary ectoderm cells 

 otherwise completely separated. I suggest that the two cells at the ends of the bridge 

 are sister cells and that the connexion between them has failed to break down. If, now, 

 myofibrils were to appear in this bridge we should be well on the way towards an internal 

 muscle derived from ectoderm, and we should have a structure comparable with the 

 muscular sphincter of the maxillary glands of a Cyprid whose development I have 

 previously worked out (Cannon, 1925). 



The meaning of the arrangement of the circular muscles across the dorsal longitudinal 

 series I think can be understood by considering what would happen if they were absent , 

 and the dorsal longitudinal muscles contracted alone. The pressure produced by the 

 latter muscles would cause the lateral parts of the body wall to bulge outwards. The 

 system of circular muscles radiating from this lateral region naturally counteracts this 

 tendency and so allows the contraction of the body wall as the means of controlling 

 the movement of fluid in the body. 



One further point may be mentioned here. In the case oiDoloria I pointed out (1931, 

 p. 447) that the dorsal body wall in its constitution is Annelidan. That is correct 

 morphologically, but only partly so physiologically. Both the Annelid body wall and 

 the Ostracodan trunk wall function in putting up and controlling the internal body 

 pressure, but whereas in the former the longitudinal muscles and the circular muscles 

 are antagonistic, in the Ostracod they work together. 



ENDOSKELETON AND ARTICULATED SCLERITE SYSTEM 



A potash preparation of Gigantocypris appears to consist of three distinct skeletal 

 systems — the shell, a ventral framework supporting the limbs, and the caudal furca, 

 all three held together by the flimsiest of cuticles. The skeletal elements within each 

 system can be studied in such a preparation by the acid fuchsin method, but by the use 

 of chlorazol black it was possible to stain the sclerites in whole preparations without 

 previous maceration in potash. The muscles stained a brownish red, while the skeletal 

 elements stained a distinct greenish grey. The extremely thin cuticle stained the palest 

 grey of an indigo shade from which the greenish tinge was absent. 



