146 THE APODOUS HOLOTHUBIANS 



are thus truly "circular" muscles, and an internal epithelium of flat, ciliated 

 cells. From the circular canal arise five radial and two interradial tubes; the 

 former become the "radial canals" of the body-wall and give rise to the ten- 

 tacle canals, while the latter are the stone-canal and polian vessel. The stone- 

 canal is always single and unbranched in the Molpadiida^; it leaves the circular 

 canal in the median dorsal interradius and lies between the two layers of the 

 dorsal mesentery; it is an irregular, twisted tube, proportionately small, running 

 forward and upward, to terminate within the body-cavity in a whitish madre- 

 pore plate or body. This madreporite is flattened on one side and more or less 

 convex on the other ; it contains a central cavity which is directly continuous with 

 the lumen of the stone-canal and opens into the body-cavity through numerous 

 pore-canals, which may be more or less branched. According to Danielssen 

 and Koren ('82), in Molpadia the madreporite is not at the end of the stone- 

 canal, but the latter terminates in the body-wall beyond it. The stone-canal 

 and madreporite consist of connective tissue covered externally with the flat, cil- 

 iated epithelium of the body-cavity and lined internally with a layer of still 

 more conspicuously ciliated cells. In the stone-canal these latter cells are low 

 and cubical on the side next the mesentery, but become much higher on the op- 

 posite side ; the cilia which they carry are about equal to the height of the cell. 

 According to Danielssen and Koren, the stone-canal of Molpadia Contains abun- 

 dant calcareous deposits, but in Caudina arenata these are wholly wanting. 

 The madreporite, however, always consists chiefly of closely interlocked, irreg- 

 ularly branching, calcareous bodies. The polian vessel, like the stone-canal, is 

 always single and unbranched in the Molpadiidng ; it leaves the circular canal in 

 the left ventral interradius. It varies considerably in size, but is always rela- 

 tively small. The histological structure is similar to that of the circular canal, 

 only the layer of circular muscle fibers is much thicker. The radial canals are 

 largest where they leave the circular canal, and become rapidly smaller by giv- 

 ing off three branches, from which the tentacle canals are formed. Each radial 

 canal runs forward on the inner or axial side of a radial piece of the calcareous 

 ring, just anterior to which it gives ofT the tentacle canals, either one on each 

 side at the same level, and then the third one further up, or all three at different 

 levels; then it bends backward along the inner or axial side of the hyponeural 

 canal and runs to the extreme posterior tip of the body, where it terminates 

 in one, three, or more rudimentary ambulacral aj^pendages. In its histological 

 structure the radial canal differs from the circular canal chiefly in the absence 

 of any circular muscles; the external covering of a ciliated epithelium is also 

 lacking after it turns backward in the body-wall, save as it is indirectly cov- 

 ered on its inner side by the lining of the body-cavity. On the side of the radial 

 vessel next to the hyponeural canal there are present some longitudinal muscle 

 fibers between the connective tissue and the epithelium lining the radial canal, 



