296 Dr. P. H. Carpenter on the 



it would correspond to the ocular plates and ambulacral 

 field in Echinus, and consists of deltoid pieces and ambulacral 

 field in Pentremites. The ventral portion perhaps also had 

 better be called actinal or interambulacral system, and would 

 correspond to the genital plates and interambulacral field, and 

 consist of basal plates and fork pieces in Pent re mites." 



The following remarks occur to one on reading this 

 passage : — 



1. The deltoid plates of a Pentremite are interradial, i. e. 

 situated between the ambulacra. They cannot therefore 

 possibly correspond to the ocular plates of an Echinus • for 

 these are situated radially and receive the distal ends of the 

 ambulacra, in very much the same way as the fork pieces or 

 radials of a Pentremite enclose a greater or less portion of the 

 outer ends of the ambulacra. 



2. " Interambulacral " is not used as a synonym for 

 " actinal " by any writer on Echinoderm morphology. 



3. If Mr. Hambach will consult the writings of Prof. Alex- 

 ander Agassiz * upon the structure of Echinoderms, he will 

 find the genital plates described as belonging to the abactinal 

 and not to the actinal system ; while Gotte's observations f 

 prove that the basal plates of a stalked Echinoderm are like- 

 wise abactinal in position, and not actinal as they are called 

 by Mr. Hambach. He is right, however, in calling (hem 

 interambulacral, and in comparing them to the genital plates 

 of Echinus. But his parallel between the interambulacral 

 field of an Echinus and the fork pieces or radial plates of a 

 Pentremite is at variance with every principle of Echinoderm 

 morphology. Like the basals, and also the genital and 

 ocular plates of an Urchin, the radials constitute a funda- 

 mental element of the abactinal, or apical system, as it has 

 been well designated by Agassiz and Loven. 



The nomenclature of Echinoderm morphology is doubtless 

 somewhat difficult ; but if the fundamental difference between 

 radial and interradial, or ambulacral and interambulacral, on 

 the one hand, and between actinal and abactinal on the 

 other, be carefully borne in mind, much that seems obscure is 

 readily understood. Mr. Hambach, however, uses these terms 

 in the loosest manner, and the result is that abactinal plates 

 in which the ambulacra of Pentremites terminate (the radial 

 or fork pieces) are described by him as actinal and interambu- 

 lacral ! Is not this rather hard upon the unfortunate student 



* ' Revision of the Echini,' Cambridge, U. S. 1872-74, p. G35 ; and 

 ' North American Starfishes,' 1877, pp. 38, 02, 93. 



t " Vergleichende Entwickeluiigsgeschichte der Comatula mediter- 

 ranean Arch. f. mikrosk. Anat. Bd. xii. 1876, p. 595. 



