38 MR. p. H. CARPENTER ON THE GENTJS ACTINOMETRA. 



(§ 25) "VVe have seen iu sect. 22 that iu certain of the arms of Actbiometra the Avater- 

 Ycssels are simple tubes, like the integumentary water-vessels of the 2Iolpad'ula;, and are 

 not in connexion with any tentacular apparatus. Whether the mouth he radial or 

 interradial, the non-tentaculiferous arms are invariably the ahoral ones ; so that in the 

 latter case they belong to the trivium (PI. I. figs. 6-15), and in the former to the bivium 

 (PI. I. fig. 5) \ This last, however, is not always the case; for I have a specimen of 

 Act. Solaris in which an anterior arm (Ci) of one of the two ambulacra of the bivium is 

 tentaculit'erous, while a posterior arm (Ei) in the trivium has no tentacles; it is never- 

 theless ahoral in position, as may be seen from PI. I. figs. 2, 5. 



In only one individual of Act. poly morpha (PI. I. fig. 15) have I found a non-tenta- 

 culiferous arm on one of the two anterior radii (A, B.) ; but this was a very remarkable 

 case. Out of 31 arms, 19 were entirely devoid of a tentacular apparatus ; and in 

 15 of these the fusion of the two sides of the ambulacral grooves had taken place either 

 on the disk or in the basal arm-segments, so that an " ambulacral nerve " was wanting 

 in nearly half the total number of arms. In the other four non-tentaculiferous arms the 

 groove remained open for a short distance, and then closed in the manner above 

 described. Three of these four arms constituted the anterior division (Ej) of the left 

 lateral ambulacrum ; but the fourth was the first arm of the left anterior ambulacrum (Aj), 

 and was borne upon the same palmar axillary as a well-developed ordinary tentaculiferous 

 ai'm. Pieces of the middle portions of these two arms are represented in PI. II. figs. 3 and 5, 

 and their terminations in figs. 4 and G. With this exception, I have invariably found the 

 non-tentaculiferous arms on the ahoral side of the disk ; their number and distribution, 

 however, vary exti-emely, not only in dilferent species but in different individuals of 

 the same species. 



Thus in Act.pohiniorpha, in Plate II. fig. 8, the former is as low as -f^ of the total 

 number of ai-ms, while in fig. 15 it reaches W- Even in two individuals with the same 

 number of arms it may be different ; thus in figs. 8 and 9 it is -/o ^^^ ih respectively, 

 and in figs. 12, 13 it is \% and \^. The individual represented in fig. 12 was also 

 remarkable for the fact that one of its ahoral arms belonging to the posterior division 

 of the left lateral ambulacrum (E,) was tentaculiferous, while those on either side of it 

 were not so. 



In all the specimens of the typo of Act. polijmorpha which I have examined, and in 

 three of its varieties, of which I have, unfortunately, only single specimens, more or 



and appears to be separated from the h3-pobla8tic epithelium liniug the water-vasciilar ring by a remnant of tho meso- 

 blastic tissue -which occupied the blastocool of the Echinopsedium. One or other of those two layers, the hypoblast lining 

 the atrium, or the mesoblast between it and the epithelium of the water-vascular ring, must give rise to the " ambu- 

 lacral nerve," which cannot bo in any way derived from the cpiblast. I am inclined to believe that the " nerve " is 

 most probably of mesoblastic origin, and that the remainder of the mesoblast (in this position) is converted into the 

 muscular layer of the ventral wall of the water-vascular ring ; whUo the blood-vascular ring is a remnant of the 

 primitive blastococl. Huxley (' Anatomy of Invcrtcbrata,' p. 5.50) has suggested a similar origin for the nerve-canals 

 (pcriha;mal canals, Ludwig) of the Asterids. 



' In all these figures (PI. I. figs. 5-16) tho tentaculiferous ambulacra are indicated by dark lines, and the non- 

 tentaculiferous grooves by fainter lines. 



