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area of an Echinus, so stretched as to cover the five arms, while on 

 the lower side we have the actinal portion, corresponding to the actinal 

 area of an Echinus, but limited to a smaller space. It is in this fur- 

 row that we find in a Starfish the two kinds of plates, the ambu- 

 lacra! and interambulacral, which constitute almost the whole of an 

 Echinus ; the perforated or ambulacral plates, however, being larger 

 than the imperforated or interambulacral plates which extend along 

 their side, and it is upon these plates alone that spines, homologous 

 to the spines of Echinus, are placed. (These spines are called Furclien- 

 papille7i by Miiller.) In Sea-urchins, both ambulacral and interam- 

 bulacral plates increase gradually from the mouth outward and up- 

 ward, until they reach the maximum diameter of the animal, and then 

 decrease again toward the abactinal area, which they surround, thus 

 forming an arch. In Starfishes the largest plates are situated nearest 

 th« mouth, diminishing gradually toward the end of the ray, thus 

 giving the animal a more or less pentagonal or star-shaped form. 

 These facts may be made use of in a more accurate description of 

 Starfishes and Sea-urchins. 



The ambulacra and interambulacra have thus far furnished the 

 best characteristics, but I propose to take equally into account the 

 abactinal area. 



This is generally crowded with spines and other appendages, but 

 these are merely external, and in making preparations to study the 

 abactinal region, I have brought out features which have thus far 

 entirely escaped the notice of naturalists, and which difier widely in 

 what had before been considered as closely related forms. The char- 

 acters given of different genera, derived from the rows of spines of the 

 ambulacral areas, are also susceptible of much more precise definition 

 than formerly. Rows of such spines have been called ambulacral 

 spines, which do not belong to this system at all, and which merely 

 have a certain resemblance to ambulacral spines, while in reality 

 they belong to a system peculiarly developed in Starfishes, having no 

 connection with the Sea-urchin type of ambulacral spines. At the 

 end of the series of ambulacral plates of the Starfishes we find an 

 odd plate, under which stands an eye-speck, and over the latter we 

 find an odd ambulacral sucker. Now, all the other ambulacral 

 suckers are arranged in pairs on each side of the ambulacral furrow, 

 and this sucker has thus far been confounded with the other ambu- 

 lacral suckers. The plate above the eye does not belong to the 

 ambulacral or interambulacral system, but to the abactinal system, 

 and the odd sucker is homologous to the tentacle found in the prolon- 

 gation of the chymiferous tubes of Acalephs. This being determined, 

 if we consider the rows of large plates on the sides of the arms in 

 such genera as Astropecten, Anthenea, Goniaster, &c., we see at once 



