404 



COMPARATIVE ANATOMY 



CHAP. 



The form and size of the calcareous ring and its separate parts vary greatly. The 



radials are often lengthened backwards (apically) into two prongs of varying length, 



between which the radial water vascular trunks run. 



It not infrequently happens that the separate^parts become partially or altogether 



broken up into single pieces, ^ihich are connected together like a mosaic. 



The number of pieces in the ring may increase or decrease. AVhere there are 



more or fewer than ten pieces, it is always the interradials which either increase or 



diminish in number. This is comprehen- 

 sible when we remember that the longi- 

 tudinal muscles of the body are attached 

 to the radials. 



The interradial portions are wanting in 

 species of the genera Phyllojmrua, Cucum- 

 aria, and Trochostoma, and in many Elasi- 

 pot/fi, especially in the whole family of the 

 Elpidiidac. 



More than ten pieces are found in many 

 Synaptidm, viz. in nearly all those forms 

 which possess more than ten tentacles. 

 The number of e.xtra interradials then 

 usually corresponds witli that of the sujier- 

 numerary tentacles. 



Six - rayed specimens] of Cncumaria 

 Planci hare been described, whose cal- 

 careous ring consists of six radials and six 

 interradials. 



The ring which is originally radiate may 

 become bilaterally symmetrical. Its plane 

 of symmetry then agrees with the general 

 plane of symmetry of the body, and passes 

 through the fifth interradins (the so-called 

 dorsal interradius in which the genital 

 aperture lies) and the central (first) radius 

 of the ventral side. The symmetry is 

 determined either by the fact that the 

 portions of the ring on the ventral side 

 differ in form, size, and manner of connec- 

 tion from those on tlie dorsal side, or else 



Fio. 3411.— Tlie CBSophagus and half the 

 oral tentacles of a dendrocMrote Holo- 

 tliurian (after Ludwlg). 1, Genital aperture ; 

 ■2, genital duct ; 3, radial pieces of the cal- 

 careous ring ; 4, retractor muscles ; 5, madre- 

 porite ; 6, stone canal ; 7, dorsal mesentery ; 

 8, intestine; 9, Polian vesicles; 10, circular 

 canal ; 11, continuation of the radial calcareous 

 pieces ; 12, proximal portions of tlie radial 



canals of the water vascular system ; 13, inter, 

 radial pieces of the calcareous ring ; 14, one of '^y the presence of a larger number of such 

 the two small ventral tentacles. portions, in consequence of an increased 



number of interradials in definite sym- 

 metrical interradii. For instance, Synajda digitata has seven interradials, one eacli 

 in the mediodorsal and in the two ventral interradii, and two each in the dorso- 

 lateral interradii. 



The portions of the calcareous ring are more or less closely united together 

 by means of connective tissue (never by means of muscles) ; in some cases they are 

 firmly fused together. 



Structures corresponding to the calcareous ring of the Holothurioidea have long 

 been sought for in the other classes of the Echinodermata. It was thought that 

 in the Echinoidea it might perhaps be represented either by the teeth or by the 

 perignathous apophysial ring, or in certain parts of the masticatory apparatus. 



The homology of the teeth of the Echinoidea with the calcareous ring of the 



