404 



COM PA RA TI VE A NA TOM Y 



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 infre(iuently 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 interradial portions are wanting in 

 species of the genera Phylloporua, Gucum- 

 aria, and Trochostoma, and in many Elo.ii- 

 po(/n, especially in the whole family of the 

 Mpidiidae. 



More than ten pieces are found in many 

 SynajMdcc, viz. in nearly all those forms 

 which possess more than ten tentacles. 

 The number of extra inten'adials then 

 usually corresponds with that of the super- 

 numerary tentacles. 



Six - rayed .specimens] of Ciicumaria 

 Planci have been described, whose cal- 

 careous ring consists of six radials and six 

 interradials. 



The ring which is originally radiate may 

 become bilaterally S3rmmetrical. Its plane 

 of symmetry then agrees with the general 

 plane of symmetry of the body, and passes 

 Fui. 34!!.— The oesophagus and half the through the fifth interradius (the so-called 

 oral tentacles of a dendrochlrote Holo- dorsal interradius in which the genital 

 thurian (after Ludwig). i, Genital aperture ; aperture lies) and the central (first) radius 

 •2 "enital duct : 3, radial pieces of the cal- ,. .■, i. i • i rni „ + 



' °="""\ / 1 , , - , .„,i^„ ot the ventral side. The symmetry is 



careous nng ; 4, retractor muscles ; 0, inadre- _ ,. ,i , ., 



porite ; 0, stone canal ; 7, dorsal mesentery ; determined either by the fact that the 

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

 canal; 11, continuation of the radial calcareous \[ff^Y jn form, size, and manner of connec- 

 pieces; 12, proximal portions of the radial ^.^^^ j.^.^^^^ ^j^^^^ ^^^ ^,^^ dorsal side, or else 

 canals of the water vascular system ; 13, uiter- i r i 



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

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



numlier 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. 



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



