VIII EGHINODEBMATA—THE INTEGUMENT 415 



arms of the Crinovha, there is no sharp line of distinction between the body 

 epithelium and the cutis. Such a distinction is, however, demonstrable in very young 

 stages. In later stages of development the elements of tlie two forms of tissue seem 

 to mingle, and skeletal substance forms right up to the surface of the integument. 



In many Holothurioidea also, the body epithelium, as such, is very indistinct. 

 In Cucumaria, for example, the cutis appears at the surface of the integument, and 

 the body epithelium is found in the form of nests of cells scattered within the peri- 

 pheral layer of the cutis. Each cell sends a thin process to the surface of the 

 integument. 



(6) The body epithelium is usually covered by a cuticle of varying thickness. 



(c) The body epithelium is ciliated over the whole surface of the body in the 

 Asteroidea and Edhinoidm, but in the Crinoidea only in the food grooves. 



The integument of the Ophiuroidea, Crinoidea (with the exception of the food 

 grooves), and Holothurioidea, is non-ciliated. 



(d) The body epithelium of the Asteroidea is rich in glands. The glands are 

 usually unicellular (goblet glands, gi'anular glands, etc. ), and remain on the level of 

 the epithelium. In Echiiuister sepositus, large multicellular glands are also found, 

 whose pear-shaped or spherical bodies dip down into the cutis. In the integument 

 of the Solothurioidea also glands have been described, and it will probably be dis- 

 covered that certain epithelial cells of the Echinoidea are of a glandular character. 



(«) The integumental pigment may belong to the epithelium as well as to the 

 cutis ; it not infrequently occurs in both layers. 



(/) Epithelial sensory cells, ganglion cells, and nerve fibres will be described in 

 another place. 



(2) The cutis of the Echinodermata is always very thick, although it shows 

 extraordinary variations in this respect according to the genus and species. It 

 everywhere consists («) of a ground- or intercellular substance of gelatinous or carti- 

 laginous consistency, and (b) of the nucleated connective tissue cells which secrete 

 this ground-substance and are embedded in it ; these cells are spindle-shajied, star- 

 shaped, etc. There are, further, (c) in all Echinodermata, granulated plasm cells 

 or wandering cells (amoebocytes) similar to those which are to be found in different 

 body fluids. These can move, like amrobse, in and through the different tissues. 



In Holothurioidea, these wandering cells may collect in such quantities in the 

 deep looser Jayer of the cutis, as to form a distinct layer CVVanderzellenschicht). 



The calcareous skeleton of the body wall of the Echinodermata always lies in 

 the cutis, whether it consists, as in the Holothurioidea, of isolated calcareous cor- 

 puscles, or, as in other Eohinoderms, of larger plates of lattice-like or spongy struc- 

 ture. In sections through the decalcified body wall, the spaces in which the skeleton 

 lay are visible. In other words, the connective-tissue fills up all the spaces in the 

 spongy calcareous skeleton. Since the wandering cells can travel to the surface 

 through these spaces, they may play an important part in the nutrition of the soft 

 parts which lie at the surface of the skeleton, especially in Asteroids and Eehinoids. 



It appears that even the intercellular substance may occasionally become differ- 

 entiated into fibres, which, however, are difficult to distinguish from the fibrous 

 processes of the connective-tissue cells. 



Where two skeletal plates are united by a suture, this suture is formed of thickly 

 crowded parallel fibres, which connect the ground-substance of one plate with that 

 of the other. 



