448 DISCOVERY REPORTS 



efficient system of endoskeletal elements and articulating sclerites to the powerful 

 adductor muscle and this, in turn, reciprocally supports the bivalve shell. The dorsal 

 body wall is thus, to a large extent, emancipated from any supporting function and 

 consequently it has lost its rigidity and become a soft flexible dome. 



In all the Crustacea the function of the dorsal longitudinal muscles is to extend the 

 caudal region. In Doloria, and probably in Ostracods generally, since they all have a 

 flexible body wall, the softening of the cuticle in this region would have placed these 

 muscles at a disadvantage and so, as a compensation, they have become attached along 

 their length to the cuticle. On contraction they can still extend the caudal region but, at 

 the same time, they must press in the body wall and so have a great effect on the move- 

 ment of body fluids. This new function of moving the body fluids or of regulating the 

 body fluid pressure, I believe, must be the main function of the dorsal body wall, and 

 to enhance it the circular muscles have developed. 



That they have developed from the ectoderm is not strange. The Crustacean ecto- 

 derm is, in my opinion, a supporting tissue of varied potentialities. It may produce a 

 hard external plate or sclerite. It may pass inwards and produce an endoskeletal apodeme 

 (Manton, 1928, p. 414), or a tendon joining transverse muscles (Manton, 1928, p. 412). 

 Or again, it may give rise to a muscular sphincter, a group of cells exhibiting con- 

 tinuous myofibrils but not striation, such as I described in Cyprids (1925, p. 15). It 

 may finally give rise to typical striated muscle which "cannot be distinguished from 

 the mesodermal muscles " (Cannon, 1926 b, p. 413). All these are cases where the actual 

 development of the ectoderm has been followed embryologically. The circular muscles 

 of Doloria I consider come between the last two cases quoted. 



BLOOD SYSTEM 



Very little is known concerning the circulatory system of Ostracods beyond the 



fact that a globular heart with a single pair of ostia occurs in Cypridinids and Halo- 



cyprids. Claus (1891) described the histology and general anatomy of the Halocyprid 



heart, and from his figures it is obvious that he was dealing with moderately well-fixed 



material. Both Miiller (1927, p. 415) and Klie (1929, p. 37) state, in addition, that a 



short vessel arises anteriorly which runs to the brain and that, in many Cypridinids, 



a pair of lateral vessels run towards the attachment of the adductor muscles (see later, 



p. 453). The lengthiest description of an Ostracod blood system is that of Gigantocypris 



by Liiders (1909). It is, however, badly illustrated and undoubtedly inaccurate (see 



later, p. 449). 



HEART AND PERICARDIUM 



The heart lies in a well-defined pericardial space close under the middle region of 

 the hinge of the shell just above the level of the eye stalks (Figs. 4, 5). The general 

 anatomy and histology of the Halocyprid heart described by Claus (1891, p. 41) will 

 apply equally to Doloria. 



The whole heart is covered, except for its apertures, by a layer of parenchymatous 



