ECHINODERMATA: ASTEROIDEA. 



203 



system, from which are given off the rows of suctorial feet, or 

 " tube-feet" 



It follows from this that the radiating vessels of the ambulacral system 

 are otitside the chain of ambulacral ossicles, so that these latter are to be 

 regarded as an internal skeleton, and they do not correspond with any part 

 of the skeleton of Echinoids * at least they do not correspond with the 

 perforated ambulacral plates of the 

 Sea-urchins. The ambulacral ossi- 

 cles, however, of the Star-fishes are 

 of such a form that by their apposi- 

 tion an aperture or pore is formed 

 between each pair. By means of 

 these pores (fig. 98, a) the tube-feet 

 communicate with a series of little 

 bladders or ' ' ampullae, " placed above 

 the chain of ossicles. These per- 

 forations, however, do not correspond 

 with the perforated plates of the 

 Echinoid test, and the tube-feet of 

 the Star-fishes pass through no "por- 

 iferous " plates on their way to the 



Fig. 98. Diagram of a Star-fish (Goniaster), 

 This may be rendered more mtel- showing the under surface, with the mouth 

 ligible by examining" a section of the and ambulacral grooves, a Ambulacral os- 

 arm of a Star-fish from which the s i cles ' the ambulacral pores between 

 , , i , ic them ; Adambulacral plates, bounding the 



soft parts have been removed (fig. ambu i acra i grooves; m Marginal plates, 

 99). In such a section the ambu- (wanting in many species) ; o Oral plates, 

 lacral ossicles (a a) are seen in the placed at the angles of the mouth. 

 centre of the lower surface, united 



along the middle line by their inner extremities. They are so placed 

 as to form a kind of elongated pent-house, and immediately beneath the 

 line where the ossicles of one side are articulated with those of the other 

 side is placed the ambulacral vessel (b}. Superficial to this, again, is a 

 nerve-cord ; so that the whole chain of ambulacral ossicles is placed in 

 the midst of the soft parts of the animal, and is thus clearly an internal 

 skeleton. At their outer extremities the ambulacral ossicles are articulated 

 by the intervention of the ' ' adambulacral plates" (fig. 98, ), with plates 

 belonging to the external or integumentary skeleton. As before said, 

 the shape of the ambulacral ossicles is such that a pore is formed by 

 the apposition of each pair ; and by these apertures each tube-foot com- 

 municates with a vesicle placed internal to the chain of ossicles. It will 

 be seen, however, that the tube-feet (indicated by the dotted lines in the 

 figure) do not pass through these apertures, or through any other pores of 

 the skeleton, on their way to the surface. The "poriferous zones" of the 

 Sea-urchins are part of the external skeleton, and are not represented in 

 the Star-fishes. On the other hand, the integumentary skeleton in the 

 Star-fishes is absent along the ambulacral areas, or along the areas occupied 

 by the ambulacral grooves. 



The circulatory system of the Asteroids is represented by a 

 group of vessels communicating ventrally with an oral ring 



* The structures in the Echinus, which are truly homologous with the 

 ambulacral ossicles of the Asteroidea and Ophiuroidea, are the so-called 

 "auriculae." 



