196 



COMPARATIVE ANATOMY 



CHAP. 



(2) of a system of fibres stretched between the invaginated wall of the proboscis 

 sheath and the neighbouring outer body wall (parieto-vaginal muscles) ; (3) of 

 circular muscles, generally developed only on the anterior body wall, though in 

 Paludkclla they appear as subdermal muscular hoops in the whole body, and by 

 their contraction cause the protrusion out of the cells of the withdrawn anterior 

 end of the body with its tentacles. All these circular muscles are to be considered 

 as remains of the circular muscle layer ; the retractors and parieto-vaginal muscles as 

 remains of the longitudinal musculature of a dermo-muscular tube. 



In the BracMopoda a typical dermo-muscular tube is as little 

 developed as in the Bryozoa ; its absence is here also evidently to 

 be referred to the development of a shell. As remains of a dermo- 

 muscular tube, there are: (1) lying under the integument of the 

 mantle, weakly -developed fibres running transversely and longitu- 

 dinally ; (2) the arm muscles (protractors and retractors) ; and (3) the 



Fig. 129.— Preparation of Waldlielmia flavescens (after Owen), seen from the right side, to 

 demonstrate the musculature, the peduncle (jj) and the calcareous framework (Z)) which serves 

 to support the arms. Dd, Dorsal, Dv, ventral shell valve ; mj, '/jij, m^, m4, muscles for opening and 

 closing the shell. 



longitudinal muscles of the p>eduncle, which, in the almost universal 

 absence of a circular musculature, are counteracted by its elastic wall. 

 In the BracMopoda a system of strong dorso- ventral muscles 

 passing through the body cavity (Fig. 129) serve for closing and 

 opening the two valves of the shell (adductors and divaricators). They 

 are attached to both the shell valves in the posterior region of the 

 body in the neighbourhood of the hinge (where this is present). 

 These muscles cannot be regarded as dislocated or modified portions of 

 the dermo-muscular tube. 



In Dinopihilus there is found under the body epithelium a very 

 weakly developed dermo-muscular tube (circular and longitudinal 

 muscle layers). 



The muscles of the Rotatoria mostly run as isolated fibres in the 

 longitudinal direction, or circularly round the body. The longitudinal 



