THE APODOTJS HOLOTHUBIANS 49 



parts of the body and in some species chiefly about the anterior end or on the 

 dorsal side. They often crowd about the calcareous ring and even occur in 

 that structure. The color is generally some shade of red, brown, or black ; but 

 purple, green, and yellow pigments also occur, and perhaps other colors. It 

 is possible that the pigment bodies sometimes crowd into the epidermis, but 

 it is doubtful if this is a normal condition. The pigment is generally, if not 

 always, insoluble in water, ether, or pure alcohol, but is rapidly discolored 

 or profoundly changed by acids, alkalies, and corrosive sublimate. 



The calcareous deposits of the body-wall are rarely entirely wanting. They 

 are formed by special mesenchyme cells in the connective-tissue layer, and 

 their manner of growth is well known. They are almost, if not entirely, pure 

 carbonate of lime. It is most convenient to classify them under four heads: 

 anchors and plates, wheels, curved rods, and miliary granules ; all four kinds 

 never occur in any one species, though three often do, and there are usually 

 at least two kinds present. 



The anchors and plates (Plates IV and V) are more or less symmetrical, 

 and always lie in the body-wall, with the anchor over or outside the plate, 

 and usually their long axes are at right angles to the long axis of the animal. 

 The plate arises as a short rod, parallel to the long axis of the animal, which 

 soon forks at each end. The forks grow and divide dichotomously, with more 

 or less regularity, but the subsequent branches do not grow with equal rapid- 

 ity. As a consequence, branches of the third and higher degrees meet 

 and fuse, thus inclosing the open spaces which are the perforations in the 

 completed plate. The edges of these holes may remain smooth, or they 

 may become toothed, although the smallest holes are always smooth. In 

 some species the branching, which gives rise to the plate, proceeds so irreg- 

 ularly that the plates become very asymmetrical, this being especially 

 true when the perforations are small and numerous. In species with symmet- 

 rical plates the number of large, toothed holes is generally 6 or 7, rarely more, 

 but the number of small, smoooth holes is not limited, though generally less 

 than 10 ; they are found mostly at the smaller end of the plate. Usually the 

 plates are not flat, but are more or less concave on the outer side, and the pos- 

 terior end frequently bears an outwardly curved bow, which thus arches the 

 concavity. This bow sometimes bears teeth at the center on the anterior edge. 

 The bow is often imperfect and frequently wanting. Anchor-plates vary in 

 size from 0.1 mm. up to 1 mm. in length, and the breadth is from one-half the 

 length up to nearly equaling it. The plates are always shorter than the 

 anchors which accompany them, though the proportion between the two pieces 

 is very variable. The plates are frequently longest at the posterior end of 

 the body. They are occasionally double and provided with two bows. Two 

 quite different sorts of plates sometimes occur in the same species. The 



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