78 THE VOYAGE OF H.M.S. CHALLENGER. 



mass traversed by foreign bodies. The epithelium lying below this deposit is without 

 cilia, and varies very much in height; the epithelial stratum sometimes shrinks to an 

 almost invisible layer, and sometimes rises into long, filamentous cylindrical cells. 



The Avail in Cereus spinosus is very thick (as much as 3 mm. in transverse section), 

 tough, and leathery as in the majority of Sagartida3. It is constricted at the upper end 

 by a circular muscle, wdiich, in spite of its strength, is entirely concealed in the mesoderm 

 of the wall. The muscle is nearly 1'5 cm. long and nearly 2 mm. broad in section at 

 its upper end, whilst it becomes narrow below as usual. It is separated both from the 

 ectoderm and the endoderm by a layer of connective tissue, 0"5 mm. broad, and without 

 muscles. Seen in transverse section (PI. VI. fig. 1), the muscular fibrdlas in the upper 

 half of the muscle form rows rising from within and below, obliquely upwards and out- 

 wards ; they are separated by broad bands of connective tissue, and placed in tiers one 

 above the other. Here and there a row consists of a single flattened primitive bundle, the 

 indentations of whose surface indicate its tendency to split up into a series of smaller 

 bundles. This process has, however, usually taken place, so that each single tier is 

 composed of a series of smaller roundish bundles and larger flattened bundles. Two 

 successive tiers of bundles are not completely separated, but connected by a network of 

 thin branched anastomosing cords ; the bundles of each tier are connected with one 

 another in the same way. The former is visible in sections taken parallel to the surface 

 of the wall, the latter in sections parallel to the base of the animal. 



The character of the muscle changes in the lower half as the bundles of fibrillse are 

 scattered at considerable distances from one another. The larger bundles are lobed in 

 transverse section, or resolved into a group of smaller bundles of fibrillae. 



The radial striation is distinctly marked on the oral disk, and is caused by the manner 

 in which the muscles are arranged, while this again is correlated with the distribution of 

 the tentacles. The radial muscular layer is ectodermal and pleated very uniformly, so that 

 the single folds of muscles are only slightly branched, and lie beside one another like 

 the leaves of a book. Besides this uniform pleating, the enlargement of the muscular 

 layer is due to the fact that the supporting layer is thinner at the insertions of the septa, 

 but becomes thickened above the middle of each interseptal space, where it forms a sharp, 

 roof-like ridge. The ridges formed in this way produce the radial striation of the oral 

 disk already mentioned ; seen from the surface, they do not project very sharply so long 

 as they are covered by epithelium, which in a measure reduces their inecpialities. The 

 ridges begin near the margin of the mouth (PI. I. figs. 4 and 5) ; they are forty-eight 

 in number, twenty-four corresponding to the intraseptal spaces of the first, second, and 

 third orders, and the other twenty-four to the intermediate interseptal spaces. The first 

 twenty-four are broadest near the margin of the mouth, become narrower towards the 

 periphery, and end on the twenty-four tentacles of the innermost row, where they run a 

 little way divided into two by a shallow furrow. The second twenty-four ridges differ, 



