COMPARATIVE ANATOMY 



CHAP. 



404 



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

 radials are often lengthened backwards (apieally) 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, which are connected together like a mosaic. 



The number of pieces in the ring may increase or decrease. Where 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 iriterradial portions are wanting in 

 species of the genera Phylloporus, Cucum- 

 aria, and Trochostoma, and in many Elasi- 

 poda, especially in the whole family of the 

 Elpidiidae. 



More than ten pieces are found in many 

 Synaptidce, viz. in nearly all those forms 

 which possess more than ten tentacles. 

 The number of extra interradials then 

 usually corresponds with that of the super- 

 numerary tentacles. 



Six - rayed specimens] of Cucumaria 

 Planci have 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 

 Fio. 34','. The oesophagus and half the through the fifth interradius (the so-called 



oral tentacles of a dendrochirote Holo- 

 thurian (after Lud-wig). 1, Genital aperture ; 

 J, genital duct ; 3, radial pieces of the cal- 

 careous ring; 4, retractor muscles ; Sjjnadre- 

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



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 



9, Polian vesicles; 10, circular portions of the ring on the ventral side 



canal ;,11, continuation of the radial calcareous differ in f ^ and manner of con nec- 



pieces ; 12, proximal portions of the radial ,. f , -, , , -, , . -, -, 



canals of the water vascular system; 13, inter- tlon frOm those on the dorsal side, or else 

 radial pieces of the calcareous ring ; 14, one of by 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, Synapta digitata has seven interradials, one each 

 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 horaology of the teeth of the Echinoidea with the calcareous ring of the 



