140 THE APODOUS HOLOTHURIANS 



2. The connective-tisfiue layer makes up the greater part of the wall, al- 

 though, as already stated, its thickness varie>s greatly. Apparently it increases 

 with age. It is thinnest anteriorly, just hehind the tentacles, but becomes 

 rapidly thicker, reaching its maximum near the middle of the body and becom- 

 ing thinner again posteriorly. As in the Synaptidip, it is made up of tibrons 

 prolongations of bipolar or stellate cells, lying embedded in a homogeneous 

 matrix. As fibers are very abundant, however, while the cells are rather infre- 

 quent, it is possible that many of the fibers have no cellular connections. Super- 

 ficially the fibers are more or less entwined among the calcareous bodies, and so 

 run in all directions; but in the deeper parts of the layer they run more or less 

 parallel to the body surface. The wandering -cells, which stain readily with 

 eosin (in Molpadia), but do not take carmine, are scattered in some numbers in 

 the connective-tissue layer and among the epidermal cells. They consist of a 

 mass of highly refractive spherules imbedded with a nucleus in a small quantity 

 of hyaline protoplasm. The shape, as is usual with amoeboid cells, is veiy varia- 

 ble. There are also pigment -cells in the conective-tissue layer of some Mol- 

 padids, although they are more commonly wanting. They are more or less irreg- 

 ular cells, with nmnerous fine branches, and contain a coloring matter, most 

 commonly purple or brown, but sometimes reddish, orange, or yellowish. They 

 are often aggregated in certain areas, thus giving rise to spots and blotches of 

 color. In most cases the pigment is not affected by pure alcohol. 



The calcareous deposits of the body-wall occur in the connective tissue and 

 are only wholly wanting in a few cases. They are most abundant posteriorly, 

 and are often present near the cloacal opening when they are not to be found 

 elsewhere. They are formed by special mesenchjTue cells, consist of almost 

 pure carbonate of lime, and first appear as simple rods or X-shaped particles, 

 which by continual growth and more or less regular dichotomous branching give 

 rise to the various kinds of deposits characteristic of the different genera and 

 species. They exhibit a very great variety of form and are accordingly diffi- 

 cult to classify, but we may for convenience group them under five heads: 

 tables, shallow closed cups, perforated plates, fusiform bodies, and anchors. 

 Commonly only one or two of these kinds occur in a single individual or species, 

 but some specimens of Molpadia exhibit all but the second. The tables (Plates 

 XI, figs. 9-11; XII, figs. 23-25, 28, 29; XIII, figs. 15-16) exhibit the greatest 

 possible variety of form and are often so irregular and grotesque as to be quite 

 unworthy of their name. In their simplest condition they consist of a I'ing 

 (forming the disc) from which three or four vertical rods arise, more or less 

 connected with each other by cross-bars (forming the spire), but in some cases 

 the ring is itself incomplete and distorted, and there may be only a rudi- 

 mentary spire. More commonly the disc is a somewhat circular plate with 4-12 

 perforations, usually symmetrically arranged and bearing a spire made up of 



