17 



ambulacra! furrows and partly surrounding the oral aperture as the so-called bueal membrane. 

 This ventral cuticle is of much finer and thinner consistency and without any trace of cal- 

 careous deposits. In the ambulacral furrows it appears as a thin transparent membrane 

 without any distinct structure; while the bucal membrane is remarkable for a high degree 

 of contractility, for which reason also it is interwoven with numerous hue concentric and 

 radial muscular fibres; it will be more particularly described hereafter. 



c. The calcareous parts belonging to the integument. 



Of the various calcareous deposits in question we may distinguish 3 principal groups: 

 1) the calcareous deposits in the interior stratum of the skin, 2) the spines issuing from the 

 surface of the body and 3) the peculiar microscopic organs found in the exterior stratum 

 of the skin, which we call pedicellarise. 



a. The interior calcareous elements of the skin. 



The formations classed in this category, which correspond to the more or less deve- 

 loped calcareous net in other star-fishes, consist of variously formed calcareous deposits in 

 the interior stratum of the skin, serving partly for the insertion of the dorsal spines, and 

 partly as a frame-work to extend the skin over the subjacent interior organs. 



In the dorsal skin of the disc, we find these calcareous parts (see Tab. I, fig. 8, 9 a) 

 in the shape of small truncated cones, situated close together and traversing the interior 

 stratum of skin in its whole thickness, yet without being connected wich each other or form- 

 ing any proper calcareous net. Each of these calcareous parts consists apparently of nume- 

 rous small cemented calcareous granules (see fig. 9 a); and on the narrower upper extremity, 

 there is always articulated one of the small dorsal spines which project beyond the skin of 

 the disc. By means of these close-lying calcareous parts, the dorsal skin of the disc acqui- 

 res a very considerable degree of solidity; although it retains its elasticity and flexibility 

 owing to their not being connected with each other. 



The calcareous deposits found in the dorsal skin of the arms present quite a different 

 appearance. These calcareous parts are however essentially confined to the basal part of 

 the arms; while the rest of the dorsal skin of the arms is quite soft and flexible, and lies 

 close to the subjacent ambulacral skeleton. But in the basal part of the arm, which con- 

 tains a number of important and some very strongly developed internal organs, (the radial 

 cseca and the organs of generation) it was necessary for the skin to have an internal sup- 

 port, in order the better to protect these organs, and to preserve them from outward pres- 

 sure. Therefore the calcareous elements deposited in the skin appear in these parts in a 

 very peculiar form especially adapted for such purpose, namely as continuous solid arched 

 transverse beams connected on each side with the ambulacral skeleton, and, being placed 



