ASTEROIDEA. 169 



The body- wall consists (1) of a single layer of ciliated columnar 

 ectoderm, with a cuticle on the outer side and a basement mem- 

 brane on the inner ; (2) of a dermis formed of a gelatinous 

 matrix containing fibrillar connective tissue and calcareous 

 plates and muscular elements ; (3) of a layer of ciliated peritoneal 

 epithelium which lines the body cavity. In the ectoderm are 

 sense-cells and gland-cells, and in some parts of the body nerve- 

 fibres and nerve-cells are present in the deeper parts of the 

 same layer. 



The integument is more or less hard and stiff owing to the 

 presence of the calcareous plates. These structures may be 

 regularly arranged and in contact with each other, or they may 

 be irregularly disposed rods forming a kind of loose network, 

 through the variously shaped meshes of which such delicate 

 processes of the body-spaces as the tube-feet and dermal branch- 

 iae project, or they may be isolated from one another. The 

 calcareous plates may be deeply imbedded in the dermis and 

 not visible from the exterior, or they may lie just beneath the 

 epidermis, so that their shape is more or less completely dis- 

 cernible in surface view. As a general rule some or all of the 

 dermal plates bear granules, or processes and spines of various 

 shapes, or pedicellariae. When these structures project from 

 the surface, as they generally do, the skin has a rough or even 

 spiny appearance ; but sometimes, especially when the plates 

 lie deep, they project but little in the fresh state, and the skin 

 is nearly smooth ; though even here, in dried specimens, the 

 skin is rough and the spines are discernible from the surface 

 (e.g. Tylaster willei, Porania, Culcita). 



In addition to these skeletal structures of the general in- 

 tegument, small calcareous pieces are found in the walls of the 

 tube-feet. 



The skeleton of the plates or ossicles falls under two heads, (a) the 

 auibulacral, (b) the ambital. 



(a) The ambulacra! skeleton consists of the ambulacral and adambulacral 

 ossicles, together constituting four rows of plates in each ray (Fig. 110). 

 They form the roof and sides of the ambulacral groove (Fig. 121). The 

 ambulacral ossicles (Fig. 119, A) are two rows of rod-shaped structures 

 which meet and are articulated together in the middle line above (Fig. 121), 

 and diverge from one another on each side, abutting at their outer ends 

 upon the adambulacral ossicles (A'). These, which correspond in number 

 with the ambulacral ossicles, though they usually alternate with them in 

 position (Fig. 119), form the edges of the ambulacral grooves and carry 



