MUSEUM OF COMPARATIVE ZOOLOGY. 47 



Interhrachial Ends of the Oral Amhulacrals. — The interbrachial 

 ends of the oral ambulacrals in the starhsh are represented in Amphi- 

 ura by two separate plates, known as the first and second pair of adam- 

 bulacrals. While in Anaphiura these plates arise from two separate 

 centres of calcification on each side of the arm (twenty centres in all), 

 and in Asterias they appear to arise directly from the ambulacrals of the 

 oral region as a single interhrachial (ten in all), their position and their 

 relation to the mouth lead me to suppose that they are really the same. 

 From the existence of a pair of spines on each oral adambulacral it is 

 necessary to suppose that two plates are consolidated in early stages in 

 Asterias. The greatest difficulty which has been encountered in sup- 

 posing the homology which I have indicated as a correct one is the fact 

 of origin from diff"erent centres of calcification in the one case, and from 

 the ambulaci-als in the other (Asterias). I am forced to admit from my 

 study of my preparations that it looks as if the account which is given 

 above of the origin of the oral adambulacrals from the ambulacrals 

 (oral) is correct, although an error may have crept in in this observation. 

 I have no doubt that the adambulacral is single (ten in all rays) from 

 the beginning. The existence of the spines in twos leads me to look 

 upon each oral adambulacral as a compound structure. 



First Interhrachial. — The homologue of this plate of Asterias in Am- 

 phiura is one of the most difficult problems connected with the whole 

 subject of the morphology of the plates of Echinoderms. It is a most 

 difficult problem to determine what plates in Amphiura correspond with 

 these plates in Asterias. The term odontophore, as others have already 

 shown, is poorly chosen to designate these plates, but as the term has 

 received a signification which is difficult to denote in any other way 

 at present, it is here retained. 



The plates which I have identified as odontophores bear the same 

 relation to the oral plates as the so called orals in Amphiura. They do 

 not, however, have a similar origin in the two. 



In Amphiura the orals originate on the border of the abactinal hemi- 

 some, and gradually grow down on the actinal side until they come to 

 occupy a position relatively the same to the adambulacral as the odonto- 

 phores to the oral adambulacrals. One of these, according to Ludwig, 

 is a madreporite. 



In Brisinga, according to Perrier, the madreporite is situated on one 

 of the odontophores. It would seem, therefore, a just conclusion, that 

 the odontophores and orals are homologous. 



If, however, we accept the theory that the orals are homologous with 



