VIII 



EGHINODERMA TA—SPH.ERIDIA 



393 



Structure of the Sphseridia (Fig. 341). — The sphteridia consist 

 (1) of a very firm and hard transparent calcareous sphere, which is 

 concentrically laminated, and does not show the lattice-like perforated 

 structure of the rest of the skeleton, and (2) of the calcareous stem, 

 which is perforated like a sponge, and is generally continued into the 

 interior of the sphere. The calcareous sphere perhaps answers to the 

 cortical layer of a large spine of a Cidaroid (cf. Fig. 337). Not infre- 

 quently the head is traversed by a canal which opens at its free end. 



The sphseridium is covered by a ciliated epithelium which is often 

 pigmented ; the waving cilia are very long at the base of the stem, 

 but gradually diminish in length towards the head. The sphseridia, 

 like the spines, are surrounded at the base where they articulate with 

 the tubercle, by a muscular envelope and by a circular ganglion, the 



l''[G. 340. — Portion of an ambulacrum 

 bordering the peristome in Echinocidaris 

 nigra, Mol. (after LovAn), magnified. 1, Spliairi- 

 dium in its niche ; 2, ambulacral double pore ; 

 3, edge of the peristome. 



Fio. 341.— Longitudinal 

 section through a sph^ri- 

 dium, diagrammatic. 1, Cal- 

 careous mass of the splut^ridiuiii ; 

 2, epithelium ; 3, calcareous 

 stem of lattice-like structure ; 

 4, muscle envelope ; -j, circular 

 ganglion ; G, tubercle ; 7, test. 



latter lying within the epithelium, which is here specially thickened. 

 The hair-like cells of this circular thickening of the epithelium are 

 probably for the greater part sensory. 



The sphseridia have always been claimed as sensory organs, and, 

 on account of their usual position near the mouth, as gustatory or 

 olfactory organs. They have also been thought to be auditory, or 

 organs for the appreciation of the movements of the water. They 

 also remind us of organs adapted for appreciating the position of the 

 body in the water. 



b. The Pedieellarise. — These are small seizing organs which rise 

 from the integument. They occur in all Ediinoidea, most Asteroidea, 

 and a few Ophiuroidea in very varying number and arrangement, and 

 in many different forms, between which, again, there are transition 

 forms. They must be considered as modified spines, or groups of 

 spines. In one and the same species various forms of pedieellarise, 



