VI. Holothurioidea. j 7 



the Dendrochirotae is constant, having the shape of an X. For it is formed along 

 the apposed edges of four prismatic hexagonal cells, and calcification gradually 

 extends over the adjacent cells, but does not include the nuclei, the presence of 

 which accounts for the openings in the calcareous corpuscles. 



According to Herouard ( 2 ) the middle or cellular zone of the body-wall in 

 Holothurians is a general lacuna containing corpuscles; and in certain parts it 

 assumes a greater importance and acquires the appearance of vessels. Along the 

 radii it is divided by the nervous layer iuto internal and external portions. These 

 radial lacunae unite into a diffused ring on the inner and posterior face of the 

 water-vascular ring. The latter communicates with the general lacunae of the in- 

 testine and with the interior of an areolar structure situated at the base of the 

 water-tube which represents the ovoid gland of other Echiuoderms. The blood- 

 lacunar system is entirely distinct both from the water-vascular system and from 

 the body-cavity. The radial nerve is divided by a layer of connective tissue 

 into two portions, internal and external. The latter joins the oral ring, while 

 the former bifurcates and supplies the retractor muscles. Branches of the radial 

 nerves also extend into the interradii and form an extensive plexus homologous 

 with that outside the test of an Urchin. 



