MUSEUM OF rOMPARATIVE Z^MiLOOY. 23 



forminiz elongated bodies whose pfreatcst lengtli lies in the direction of 

 the interradius, or parallel to it. As their size increases, four spines 

 form on each of these plates, two on the aboral and two on the adoral 

 ends The existence of these spines would indicate that they are con- 

 solidated interambulacrals, and that interambulacrals as well as ambu- 

 lacrals enter into the formation of the oral ring of calcareous bars. 



First InterbrcichiaL* — In very eai'ly conditions in the growth of the 

 oral rino- of bars, before the increase in size of the interambulacral ends 

 of the circuniorals, and before spines appear, there form in the inter- 

 brachial radii certain round or heart-shaped plates, which are thought 

 to be the first sign of the odontophore. 



These plates, ib. lie on the abactuial side of the adambulacral circuni- 

 orals, in a space between them and the genitals, and on the adoral side 

 of the cii'cumorals. Their first form is round, or heart-shaped. As the 

 growth goes on they are pushed moi'e to the aboral region of the inter- 

 radius, but never extend beyond the cover of the interambulacral ends 

 of the circumorals, by whicli, in the increased compactness in growth 

 of the calcareous network, they are almost wholly concealed when the 

 starfish is seen from the actinal side.t This is the first time that the 

 odontophore has been described in starfishes as young as Plate II. fig. 4. 

 From their position of formation they seem to be homologous with 

 hiterbrachials, which will bo subsequently described. 



The Ambulao'al Rafters. % — Under the name of ambulacral rafters 

 all actinal jjlates of the rays, with the exception of the circumorals and 

 the adambulacrals, will be included. The following plates are present 

 when the first pair of ainbulucrrds begins to form : dorsocentral, dc, 

 five genitals, ^7^-//, five terminals, t'^-t^, and ten circumoral ambula- 

 ci'als, am. In addition to these the interambulacral circumoral and the 

 odontophore, z6, are formed before the second pair of ambulacrals, am. 

 Tiie first of the median doi-sal row of plates, d, appear just after the 

 second pair of ambulacrals. The marginals, intei-ambulacrals, and second 

 median dorsal appear after the third pair of ambulacrals. 



* Tlie name odontophore, with which this is liomologous, as pointed out by Lud- 

 wig. is ill}' suited for tlie first interbracliial plates on the actinal liemisome. The 

 true name of tliese plates can hardly be known until there is some uniformity of 

 opinion as to their homologies. Tlie term interbracliial does not commit us to 

 the tlicory that they are homologous with orals or with unpaired marginals. 



t In certain deep water Asterids, according to Sladen, a part at least of tiie 

 odontophore is visible on the actinal surface of the adult. In very young speci- 

 mens of Asterias the same tiling is true. The feature in the deep-water starfishes 

 would seem to be embryonic. 



} The plates referred to are commonly called ambulacrals. 



