].">6 THE APODOUS HOLOTHURiaNS 



extended, but is much more slender. The smallest species {Eupyigus pacificus) 

 is only 6 or 7 mm. long, but the diameter is one-half as much. The great ma- 

 jority of the species, however, approach the maximum rather than the mini- 

 mum extreme, and few Molpadids, when fully mature and normally extended, 

 are less than 50 mm. long. 



CoLOE. — The usual ground color in this family is pale gray, which may 

 be unmarked and even untinted, but is usually^ shaded with brown, dull red or 

 rosy, and is often spotted or blotched with red, brown, or purple of some 

 shade. The ultimate color effect depends in most cases on the degree of de- 

 velopment of the deposits in the skin, for the occurrence of real pigment is un- 

 usual. In Acaudina, however, the color is very dark, although there are no 

 colored deposits, and in some species of Molpadia a purple, or less commonly 

 an orange or a brown, pigment is present, which more or less considerably 

 modifies the color. The excessive development of calcareous deposits in a thin 

 skin may give rise to an almost jiure white surface, while a similar excessive 

 development of the phosphatic deposits (see p. 14.'>) may make the surface ap- 

 pear deep red, purple, brown, or nearly black; in these latter cases the oral 

 disc and tail are almost always white by contrast. All intergradations occur, 

 even in a single species, between very light color and very dark; in such cases 

 there is reason to believe the older the individual the darker it becomes. Ex- 

 cept Acaudina, species lacking phosphatic deposits are seldom dark or notice- 

 ably colored, but occasionally a species is met with which has a distinct color. 

 Such, for example, is Aphelodactyla molpadioides (Plate III), which Semper 

 figures of a uniform purplish red, and its near relative A. punctata, which is 

 blotched with orange. The color is seldom different dorsally from what it is 

 ventrally, but occasionally the upper side is darker. It is not unusual for the an- 

 terior end to be more heavily blotched and shaded than is the posterior i)or- 

 tion of the animal. The colors seem to be little affected by alcohol or other 

 preserving fluids, except that rosy shades are commonly lost, and the reds, 

 browns, and purples are deepened. The body-wall may be quite thin and 

 translucent, and then the longitudinal muscles show as five deeper bands; but 

 alcohol makes it opaque, and the longitudinal muscles may then be concealed, 

 though their position is usually indicated by the furrows in the body-surface, 

 caused by the contraction of the transverse muscles. 



BoDY-suBFACE. — The surface of the body in the Molpadiidte is often quite 

 rough, on account of the calcareous deposits in the skin, but where these are 

 inconspicuous or lacking, the surface is commonly smooth and may be even 

 slimy. Verruca;, such as occur in the Synaptida^, and pedicels and papillae 

 are i)ractieally unknown. Very rarely, long, slender papillae occur in the mid- 

 dorsal interambulacrum and hardly more common is it for rudimentary pedicels 

 to occur near the ends of the body. Gland cells occur in the skin, most com- 



