MUSEUM OF COMPARATIVE ZOOLOGY. 25 



tremities of the oral ambulacrals, a single interbrachial resembling the 

 first (odontophore), but abaxial to it (PI. V. fig. 8, ib 2 ). This is called 

 the second interbrachial, ib 2 . Other interbrachials outside (abaxially 

 to) this were found ; but in the genus Asterias the number and devel- 

 opment of these interbrachials is not as great as in some other genera. 

 The first of these interbrachials, " odontophore," might be regarded as 

 homologous with the orals of the Amphiura. The homology of the 

 others in Ophiurans is not clear to me. They are of course represented 

 in other starfishes, where they are sometimes very greatly developed, 

 imparting a characteristic form to the body, filling in the whole inter- 

 brachial region. 



The remaining interbrachials may be numbered among early plates to 

 form in the young starfish. They are, however, the last plates to form 

 of all those which we have mentioned in our account of the early or 

 primary plates of the body. 



AYhen the stai-fish of a stage like that shown in Plate V. fig. 7 is seen 

 from the actinal region, an irregular triangular interval is seen in the 

 interradius just outside the two interambulacral circumoral plates, amd. 

 This interval is bounded by the adambulacral circumoral, amd, the first 

 interambulacrals, ad 1 , one on each side, and the laterals, also one on 

 each side. In the centre of this space, on a line opposite the middle 

 of the interambulacrals, the first interradial or interbrachial takes its 

 rise. As the starfish matures, other interbrachials also form outside, 

 aborally from that which has already appeared. 



Connectives. — Under this name are included certain plates of the 

 body and disk of the starfish, which bridge the intervals between the 

 others, but which have a secondary place as compared with primary 

 plates. There are connectives on the abactinal region of the arms, and 

 others on the disk, but in either case they do not differ greatly from 

 each other. In the connectives we have a multiplicity of calcareous 

 plates, imparting a compactness to the abactinal hemisome. Their 

 form, size, and number are variable, and their morphological importance 

 of a subordinate character. 



Spines. — The study of the primary spines of the young Echinoderm 

 is one which in most accounts of the development is not given very great 

 •prominence, yet these bodies are in many genera among the first calci- 

 fications to appear, antedating in formation many plates which play a 

 most important part in the determination of the external form of the 

 animal. "When the first spines appear in the starfish, there are only 

 eleven plates present, five terminals, five genitals, and a dorsocentral. 



