THE APODOUS HOLOTHURIANS 



43 



white, brown, or red are often quite thickly scattered over the body. Fine 

 lines of a darker shade often ornament the skins of gray and brown species, 

 and occasionally longitudinal stripes are present. Large irregular blotches of 

 darker or lighter shades often variegate the color. A few species are colored 

 more darkly on one side than on the other, and this is probably due to their 

 manner of life. Subterranean species are usually white or nearly colorless, 

 while species which live about reefs are apt to be gray or brown, and those 

 which live among living corals are generally the bright-colored forms. The 

 color is due, at least in large part, to the presence or absence of pigment cells 

 in the connective tissue of the body-walls. These cells are irregularly radiant 

 in shape, and may be scattered or closely crowded. In Synaptula hydriformis 

 the green color is due in some measure to unicellular algae in the connective tis- 

 sue. The chemical composition of the pigment varies considerably ; in some 

 species pure alcohol does not affect the color, while in others the pigment is 

 dissolved by it; acids and corrosive sublimate dissolve or modify the pigment 

 rapidly. White spots are very frequently due to the aggregations of mlliary 

 granules or other calcareous bodies in the skin. Complete absence of pigment 

 tends to make the body-wall transparent, and the longitudinal muscles then 

 stand out as five prominent opaque bands. Even in dark-colored species, the 

 position of these muscles is often indicated by a darker (or lighter) longitudi- 

 nal stripe. 



BODY SURFACE. The surface of the body in the Synaptidae is almost always 

 more or less rough, and is seldom smooth and slimy, though there are glands 

 in the epidermis. The roughness may be due to the small, usually crowded, and 

 regularly arranged projections of the body-wall itself, known as verruca, or to 

 the calcareous bodies, single or in heaps. Verrucse vary greatly in size and 

 prominence, according to the size of the animal and the amount of contraction 

 of the body-wall. In the larger species, they may be very prominent, 5 or 6 

 mm. high, and 6 or 8 in diameter; in such cases, they generally form regularly 

 transverse and longitudinal rows, the latter usually 10 in number. These large 

 verrucas are by no means constant in size and form and seem to be dependent 

 on the condition of the animal. They may be large, small, or wholly wanting in 

 individuals of a single species (see Plate I), and, in Euapta lappa at least, in 

 a single individual at different times. It is probable that their presence is 

 due to unusual muscular contraction brought on by abnormal external condi- 

 tions. Their presence or absence is therefore not a character which is of any 

 taxonomic importance. True verrucse are much smaller and usually much 

 more numerous, sometimes as many as 500 to the square centimeter. They 

 usually contain calcareous deposits and oftentimes pigment. The largest and 

 most noticeable verrucae occur in the genera Synapta and Euapta, but the 

 smaller ones are found in Leptosynapta, Synaptula, and Taeniogyrus. They are 



