ME. P. H. CAEPENTEE ON THE GENUS ACTINOMETRA. 447 



entirely absent. This condition, whicli is limited in A. rosacea to 

 the oral pinnules, sometimes exists in whole arms, and in all the 

 pinnules borne by them in some species of Actinometra : even in 

 the arms which come off from the anterior or oral side of the disk, 

 the ambulacral groove does not give off regular branches to the 

 pinnules borne by the third and successive brachial segments ; 

 but a variable number of these first pinnules, sometimes only three 

 or four, sometimes as many as forty, resemble the oral pinnules in 

 this respect, their ventral surface being convex and devoid of any 

 ciliated epithelium or subepithelial band, while their water-vessel 

 is simple without any lateral extensions to respiratory leaves and 

 tentacles. In these oral arms, however, branches of the ambula- 

 cral grooves enter the pinnules sooner or later, so that the termi- 

 nal ones are always provided witb a distinct tentacular apparatus, 

 while the floor of their median groove is of the usual character, 

 consisting of a ciliated epithelium and a subepithelial fibrillar 

 band. "W e have already seen 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 peristom, and that their 

 tentacles diminish and finally disappear : at the same time the 

 floor of the groove becomes very much reduced, its epithelial 

 layer thinner and thinner, and the subepithelial band almost in- 

 visible, until, in those cases in which the sides of the groove meet 

 and unite, the ciliated epithelium and subepithelial band or am- 

 bulacral nerve disappear altogether, as in the oral pinnules. Con- 

 sequently, when this union takes place on the disk, ivhole arms 

 must he entirely devoid of any nervotis supply, unless we admit 

 the nervous nature of the antiambulacral axial cords. 



In such cases it would naturally be expected that these poste- 

 rior arms would not perform the regular swimming-movements, 

 like those anterior arms which have an open tentacular groove 

 and a subjacent " ambulacral nerve ;" but Professor Semper, who 

 has kept ActinometrcB in his aquaria for weeks together, has in- 

 formed the author that he never saw the least trace of any irregula- 

 rity in the alternating movements of their arms when swimming. 



If we suppose, with Ludwig*, that the subepithelial band is the 

 sole structure of a nervous nature in the whole Crinoid organiza- 

 tion, it is difficult to understand the fact (which Ludwig himself 



* ' Morphologische Studien an Echiuodermen. I. Beitrage zur Anatomie der 

 Crinoideen,' Leipzig, 1877, p. 81 (Separat-Abdruck aus der Zeitschrift fiir 

 wisseuschaftliche Zoologie, Bd. xxviii.). 



