438 



COMPARATIVE ANATOMY 



CHAP. 



The vigorous movements of the ciliated urns no doubt serve to promote the 

 streaming and circulation of the fluid in the body cavity. 



2. Eehinoidea. In the Echinoids, the body cavity is partitioned 

 in a manner similar to that described for the Holothurioidea, by mesen- 

 teries which follow the intestine 

 in its windings (see p. 480), and 

 attach it to the inner surface of 

 the test. The genital organs 

 are also attached to the test by 

 mesenteries. In regular Echin- 

 oids, the mesenteries are much 

 perforated, but are only slightly, 

 if at all, broken through in the 

 Spatangoida. 



In this latter order, where 

 the mesenteries have to carry 

 the heavy intestine, filled with 

 sand, they are specially strong 

 and tough. The coils of the 

 intestine are here also united 

 inter se by mesenteries. Special 

 bands attach the intestine to the 

 apical and oral poles of the test, 



Fir 368 -Ciliated urns of a Synaptid (after interna l processes or apophyses 



Cuenot). 1, mesentery ; 2, circular muscle layer , . . , i j * 



of the body wall ; 3, ciliated inner epithelium of being Sometimes developed lor 



the urn; 4, endothelium of the body cavity; the attachment of the bands. 



Two ^ h apophyses are found 

 at the apical pole, at the end of 

 the stone canal, and a third not infrequently occurs at the peristome, 

 in an interradius. 



The axial sinus, with the axial organ and the stone canal, is at- 

 tached by bands on the one hand to the apical pole, and on the 

 other to the oesophagus. 



For a description of the calcareous pillars, septa, etc., which, in the Clypeastridce, 

 traverse the cavity of the test, see p. 405. 



In the fluid of the body cavity in Echinoids there are found, besides blood cor- 

 puscles, great numbers of spermatozoa-like cells, with long flagella in vigorous move- 

 ment. These may set up currents in the fluid of the body cavity. 



3. Asteroidea. The body cavity of the disc is not spacious, the 

 greater part of it being filled by the large digestive sac. Mesenteries 

 are wanting in the greater part of the intestine, or are only developed 

 as isolated filaments or strands of connective tissue. In the peripheral 

 portion of the disc, radially placed bands or septa traverse the body 

 cavity vertically in the interradii, connecting the dorsal (apical) body 

 wall with the ventral (oral) wall. 



