MUSEUM OF COMPARATIVE ZOOLOGY. 23 



forming elongated bodies whose greatest length 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 woidd 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 Interbraehial.* — In very early conditions in the growth of the 

 oral ring of bars, before the increase in size of the interambulacral ends 

 of the circumorals, 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 odoutophore. 



These plates, ib, lie on the abactmal side of the adambulacral circum- 

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

 of the circumorals. Their first form is round, or heart-shaped. As the 

 growth goes on they are pushed more to the aboral region of the inter- 

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

 of the circumorals, by which, 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 

 interbrachials, which will be subsequently described. 



The Ambulacra!' Rafters.% — Under the name of ambulacral rafters 

 all actinal plates 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 ambulacrals begins to form : dorsocentral, dc, 

 five genitals, g 1 — g 6 , five terminals, t 1 - 1 5 , and ten circumoral ambula- 

 crals, am. In addition to these the interambulacral circumoral and the 

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

 The first of the median dorsal row of plates, d, appear just after the 

 second pair of ambulacrals. The marginals, interambulacrals, and second 

 median dorsal appear after the third pair of ambulacrals. 



* The name odontophore, with which this is homologous, as pointed out by Lud- 

 wig, is ill}* suited for the first interbraehial plates on the actinal hemisome. The 

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

 opinion as to their homologies. The term interbraehial does not commit us to 

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



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

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

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

 would seem to be embryonic. 



1 The plates referred to are commonly called ambulacrals. 



