46 BULLETIN OF THE 



phiura. Undoubtedly a few of the larger might be compared, but, as 

 the connectives are not primary plates, they vary very greatly in num- 

 ber among different genera of starfishes. Moreover, they appear late 

 in development, even after generic features are acquired. A notice 

 of their modification is made under my comparison of Asterias with 

 other starfishes. Not enough is at present known of the smaller plates 

 of Amphiura to attempt any comparison of them with the connectives 

 of Asterias. 



Oral Ambulacrals. — The oral ambulacrals early take the form of the 

 " spoon-shaped " plates of Amphiura. In their early condition they 

 approach more nearly the form of the ambulacrals of Amphiura than 

 any of the ambulacral rafters of Asterias, which leads me to think that 

 they are really ambulacral in their nature. As gi'owth goes on, the oral 

 ambulacrals lose their resemblance to the spoon-shaped plates, and re- 

 semble more closely those interambulacral plates which later appear in 

 the starfish arm. The early resemblances which have led me to regard 

 the oral ambulacrals as the same as the " spoon-shaped plates " are : 

 1. Their position and number. 2. Their elongation in early stages 

 parallel with the water tube. 3. Their time of formation. The only 

 one of these three reasons which lacks observation is the third. "While 

 we know that in Asterias the first plates of the actinal region to form 

 are the circumoral ambulacrals, we do not yet know this for the spoon- 

 shaped plates of Amphiura. I believe that they are the first, but 

 cannot as yet definitely state this to be a fact. 



The later resemblance to interambulacral plates is caused by their 

 growth to the interradial regions and the presence of spines. Notwith- 

 standing this resemblance to adambulacral plates of the oral ambula- 

 crals in the interradii, Asterias is classified as a starfish with an ambula- 

 cral mouth. In starfishes in which the interambulacrals enter into the 

 formation of the mouth as well as the ambulacral, it may be supposed 

 that the growth of the adambulacrals of the mouth has prevented the 

 ambulacral from pushing into the interradii. These are possibly inter- 

 mediate, as far as the mouth goes, between Asterias and Amphiura. In 

 Amphiura the oral ambulacrals, even in late stages, are kept in the con- 

 dition of spoon-shaped plates similar in relative position to the water 

 tube to embryonic ambulacral orals of Asterias. The retention of a bi- 

 serial arrangement of the feet in Asteroids with adambulacral mouths 

 may be correlated with the growth of the adambulacrals in early stages. 

 Like the structure of the mouth parts, the rows of legs are biserial, 

 as in Ophiuroids. 



