8 Prof. J. Miiller on the Structure of the Ecliinodcrms. 



the Holothurm the ambulacra pass from the mouth to the oppo- 

 site extremity ; when they cease before attaining this extremity, 

 there is formed, opposite to the ambulacral zone, an antamhu- 

 lacral zone, e. g. the ajjex of the Sea-urchins, which is continuous 

 with the inter-ambulacral areje. 



If, like the Asteridce, an Echinoderm has radial arms, the 

 antambulacral area is continued, as well upon the antambulacral 

 side of the arms, as between them into the inter-ambulacral arose. 

 It is not uncommon, as in many Asteridce, for the inter-ambu- 

 lacral part of the perisoma to be distinguished from the antam- 

 bulacral areffi by the character of its plates — the inter-ambulacral 

 plates. The boundary between the inter-ambulacral and antam- 

 bulacral arese is at times indicated by a peripheral margin, or 

 even by special marginal plates, such as the marginal plates of 

 the pentagonal Astror/onium and Goniodiscus, between the ventral 

 and dorsal surfaces, which also lie between the ambulacral and 

 antambulacral zones. In the Asterida and many Crinoids, the 

 ambulacral or ventral and the antambulacral or dorsal, sides are 

 about equally developed. Arms are free radii with an ambu- 

 lacral and antambulacral side, and are either simple or divided. 

 Divisions produced by processes or iusections of the periphery 

 of the Echinoderm are lobes, if they present no distinction of an 

 ambulacral and antambulacral side, such as the lobes of a few 

 flat Sea-urchins, Runa, Rotula, Encope. There exist ambulacral 

 and inter-ambulacral lobes {Runa). Ambulacral lobes are ambu- 

 lacral upon both sides and may be subdivided [Rotula). Segmen- 

 tation of the margin and the formation of lobes, therefore, do 

 not approximate the sea-urchin to the star-fish. If the antam- 

 bulacral area of the Echinoderm grow up close to the mouth, 

 arms for the ambulacra may still be developed in its neighbour- 

 hood, as in Agelocrinus, Pseudocrinites, and as in the oral arms 

 of the calyx of Echinoencrinus and Echinospharites discovered 

 by Volborth. The existence of ambulacral pores iipon the ant- 

 ambulacral area, on the opposite side of the arms, is self-contra- 

 dictory ; and it would appear that wherever arms are developed, 

 whether at the circumference or at the oral part of the calyx, 

 no ambulacral pores nor suckers can exist in the region which 

 extends from the apical end to the arms. 



The development of the ambulacra, as it is here set forth, 

 renders it probable, therefore, that the pores in the plates of the 

 calyx of Caryocrinus, Hemicosmites and Echinospharites cannot 

 possibly be ambulacral pores, inasmuch as they are situated in 

 the antambulacral region and behind the arms, to which they 

 have no relation whatsoever. Pentacrinus contributes analogi- 

 cally to this view, as it possesses similar pores in the inter- 



