Prof. J. Miiller on the Structure of the Echinoderms. 243 



arms of Cupressocrinites, pass into marginal ambulacral plates, 

 which are erect plates along the sides of the ambulacral grooves, 

 and may occur as well along the ambulacral grooves of the calyx 

 as of the arms and pinnula [Pentacrinus). They are set so 

 closely upon the arms, that many occur upon a single joint. 

 Pentacrinus possesses both marginal plates dmd pinnula. Spines 

 or bristles are unarticulated appendages of the arms, and occur 

 only in the division of Crinoidea costata {Saccocoma) ; they are 

 arranged in pairs and opposite, upon each joint of the arms, 

 thus diiFering from the pinnulse, which are articulated and alter- 

 nate. Cirri are articulated processes on the stem of the Crinoids 

 and on the terminal knob of the Comatula. 



The ambulacra of the recent Crinoids are treated of in the 

 memoir upon Pentacrinus ; they are grooves which are continued 

 from the mouth upon the perisoma of the calyx towards the 

 arms and pinnules, covered by a soft membrane, and in Penta- 

 crinus supported upon each side by perpendicular calcified mar- 

 ginal plates. Within the grooves there are two series of fine 

 apertures upon which the minute suckers are seated. Upon the 

 arms and pinnulce the calcareous formations are limited on the 

 ventral surface to the marginal plates of the ambulacral grooves. 

 On the calyx, on the other hand, the ambulacral grooves are 

 supported by calcareous deposits in addition to the marginal 

 plates. Those plates which form the edges of the ambulacral 

 grooves have a wall-like elevation, and serve not only to embrace 

 the ambulacra but to support the erect marginal plates ; they may 

 be called the lateral ambulacral plates ; they are distinguished, 

 like the marginal plates, from the other ventral plates by the 

 absence of the problematical calycine pores which characterize 

 the latter. Beneath the soft covering of the groove little plates 

 also He, which were indicated in the memoir upon Pentacrinus. 

 For the sake of comparison with the ambulacral plates of the 

 Sea-urchins and Starfishes, I thought it important to subject 

 these subambulacral plates to a closer examination. They form 

 a single and therefore azygos series under the membrane of the 

 groove, and are united to the lateral ambulacral plates by a firm 

 membrane, in which lie the ambulacral pores. These pores are 

 usually situated between the lateral series of plates and the me- 

 dian series. A semicanal is excavated upon the upper surface of 

 the median series of plates, which appears destined to receive the 

 ambulacral vessel. The ambulacral vessel would therefore, as in 

 the Asterid<e, lie upon the outer surface of the ambulacral ske- 

 leton, and, as in them, be covered by the soft skin of the ambu- 

 lacral cleft, while the ambulacral pores which are connected 

 with the feet should perhaps be interpreted as passages leading to 

 ampullae. The presence of calcareous plates renders microscopical 



16* 



