36 ME. P. H. CAEPENTEE ON THE GENUS ACTINOMETEA. 



three or four, sometimes as many as forty) resemble in this respect the oral pinnules, 

 their ventral surface being convex, and devoid of any ciliated epithelium or subepithelial 

 baud ; while their water-vessel is simple, without any lateral extensions to respiratory 

 leaves and tentacles. In these oral arms, however, branches of the ambulacral groove 

 enter the pinnules sooner or later, so that the terminal ones are always provided with a 

 distinct tentacular apparatus, while the floor of their median groove is of the usual cha- 

 racter, consisting of a ciliated epithelium and a subepithelial fibrillar baud. 



We have seen in sect. 22 that in many cases the ambulacral grooves going to the 

 aboral arms become less and less distinct as they get further and further from the peri- 

 stome, and that their tentacles diminish and finally disappear. At the same time the floor 

 of the groove becomes very much reduced in extent, its epithelial layer thinner and 

 thinner, and the subepithelial band almost invisible, until, in those cases in which the 

 two sides of the groove meet and unite, the ciliated epithelium and subepithelial band 

 disappear altogether. Consequently, when this union takes place on the disk, whole arms 

 are entirely devoid of any nervous supply, if we suppose, with Ludwig, that the anti- 

 ambulacral axial cords are not of a nervous nature, and that the " subepithelial bands " 

 are the only nervous structures in the arms. In such cases it would naturally be 

 expected that these arms would be incapable of performing the regular swimming-move- 

 ments like those in which there is an open ambulacral groove and a subjacent " ambu- 

 lacral nerve;" but Professor Semper, who has kept Actinometrae in his aquaria for 

 weeks together, informs me that he never saw the least trace of any irregularity in the 

 alternating movement of their arms while swimming. 



The gradual obliteration of the ambulacral grooves by the approximation and fusion of 

 the elevated folds of perisome at their sides, which may occur to so great an extent in 

 Actinometra, is found also at the ends of the arms and pinnules of Aiitedon Eschrichtii. 

 Ludwig states (p. 75) that their terminal segments have no ambulacral groove or 

 tentacles ; and he gives a figure of a section through the end of a pinnule (pi. xiii. 

 fig. 12), the ventral surface of which is convex, while there is no ciliated epithelium 

 or subepithelial band (ambulacral nerve), although in the text Ludwig makes no 

 mention of their absence. I have found the gradual obliteration of the groove in 

 these cases to take place in precisely the same manner as in Actinometra, the only 

 difference being that the point at which the sides of the groove meet and fuse is much 

 further from the disk in the one case than in the other. 



If we suppose, with Ludwig, that the subepithelial band is the sole structure of a 

 nervous nature in the whole Criuoid organization, it is difficult to understand the fact, 

 which Ludwig himself admits (p. 10), that it gives off no branches except those which 

 go to the tentacles. It is true that in the Ophiuridea the ambulacral nerve does give 

 off branches which go to the muscles, besides those proceeding to the tentacles, as 

 described by Lange \ Teuscher -, and Simroth - ; but the researches of the first-mentioned 

 observer render it very doubtful whether the representative in the Ophiuridea of the 



1 " Beitr. z. Auat. und Histiol. d. Astericn und Opkiuren," Morphol. Jahrb. ii. Heft 2, p. 241. 



2 " Bjitr. (fee, II. Opliiurida?," Jenais. Zeitsc.h. x. p. 274. 



8 " Ar.at. und Scbizogonie der Ophiactis virens, Rars," Zcitsch. f. wissensek. Zool. xxvii. p. 473 



