ASTEROIDEA. 



179 



left posterior body-cavities of the larva is a perivisceral space 



in relation with the stomach, rectum and pyloric caeca : it is 



a continuous space, is found in the disc 



and in the arms on the abactinal side of 



the ambulacral ossicles and is traversed 



by certain mesenteries which have already 



(p. 177) been described It is lined 



by a ciliated epithelium, and contains an 



albuminous fluid with floating amoeboid 



cells. 



The papulae or dermal branchiae (Fig. 

 129, 13} are thin projections of the body- 

 wall found principally on the abactinal 

 surface of the disc and arms and con- 

 taining prolongations of the perivisceral 

 cavity. Round the bases of the papulae 

 there is generally present in the body- 

 wall a space lined by an epithelium and supposed to be 

 developed as a diverticulum of the perivisceral cavity. These 



FIG. 130. Diagrammatic 

 transverse section through 

 the axial sinus of a starfish 

 (from Ludwig). a, b the 

 lamellae of the inter- 

 brachial septum ; C axial 

 organ ; St stone -canal ; 

 H axial sinus. The upper 

 side of the section is the 

 body-wall side. 



Flo. 131. Diagram showing the arrangement of the perihaemal spaces, etc., of a starfish, 

 seen from the aboral side (after MacBride). a' axial sinus and inner perihaemal ring ; ab 

 aboral circular sinus; gen.r generative raehis ; ov.g axial organ; ph. I. II . . . ph.IV.V 

 the five parts of the so-called outer perihaemal ring; rky vestige of right hydrocoel; 

 1-7 the arms numbered. 



