202 



COMPARATIVE ANATOMY. 



skeleton is merely represented by separate and microscopic deposits, 

 these are generally definite in shape, and characteristic of genera 

 and species. 



The calcareous skeleton of the larva forms an organ of support, 

 which is generally made up by a framework of delicately attached, 

 and often perforated rods. They are ordinarily found in the larval 

 Echino'ida and Ophiurida; there are also calcareous bodies in the 

 larvae of the Holothuroi'da. The presence of a calcareous skeleton in 

 the larvas is clearly an instance of an arrangement which is common 

 to the group ; but it must not be forgotten that this larval skeleton 

 corresponds to the form of the larva, and not to that of the adult 

 Echinoderm ; none of it passes permanently into the adult form. 

 There is, in fact, a repeated change of the calcareous skeleton in the 

 Holothuroi'da. 



§ 164. 



As regards the special characters of the dermal skeleton, the 

 presence of pieces, movably connected with one another, on the 



ambulacra! surface of the arms, 

 is characteristic of the Aste- 

 roi'da. Transversely - placed 

 pairs of calcareous pieces, 

 which gradually diminish in 

 size, are found from the mouth 

 as far as the tip of the arm 

 (Fig. 100, A w) ; they form 

 the floor of a groove — the 

 tentacular groove. The sepa- 

 rate pieces form a jointed 

 series by their articular at- 

 tachments, and the suckers 

 pass out between the solid 

 joints (p). These calcareous 

 pieces are consequently known 

 as ambulacral plates. But 

 as special soft parts (ambu- 

 lacral canal and nerves) are 

 also embedded in this groove, 

 the jointed segments do not 

 appear to be purely dermo- 

 skeletal parts. The ambu- 

 lacral groove is covered by the 

 integument, which is con- 

 tinued laterally on to the am- 

 bulacral plates. It consists 

 largely of a layer of long 

 cylindrical cells, covered by a 

 cuticle. At the side it passes into a layer of cells, which is placed 

 much deeper. At the lateral edges of the groove the skeleton is 



W\ 



Fig. 102. Body disc of an Ophiurid 

 (Ophiothrix f r ag i 1 i s) ; seen from the 

 oral surface ; the bases of the arms be- 

 set with spicules may be seen (magnified). 

 C Body disc. B Arms, t Calcareous plates, 

 which coyer the canal which corresponds 

 to the tentacular groove of the AsteroYda. 

 Genital clefts, d Masticatory plates. 



