38 MR. P. H. CARPENTER ON THE GENUS ACTINOMETRA. 



(§ 25) We have seen in sect. 22 that in certain of the arms of Actinometra the water- 

 vessels are simple tubes, like the integumentary water-vessels of the Jfolpadhlce, and are 

 not in connexion with any tentacular appar/atus. "Whether the mouth be radial or 

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

 latter case they belong to the trivium (PI. I. figs. 0-15), and in the former to the bivium 

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

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

 tentaculiferous, while a posterior arm (EJ in the trivium has no tentacles ; it is never- 

 theless aboral in position, as maybe seen from PI. I. figs. 2, 5. 



In only one individual of Act. polymorpha (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 cither 

 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 (E 2 ) of the left 

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

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

 arm. 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 aboral side of the disk ; their number and distribution, 

 however, vary extremely, not only in different species but in different individuals of 

 the same species. 



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

 number of arms, while in fig. 15 it reaches Jf. Even in two individuals with the same 

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

 and in tigs. 12, 13 it is ^f and |f . The individual represented in fig. 12 was also 

 remarkable for the fact that one of its aboral 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 type of Act. polymorpM 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 hypoplastic epithelium lining the water-vascular ring by a remnant of the meso- 

 blastic tissue which occupied the blastoccel of the Echinopoedium. One or other of these 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 be in any way derived from the epiblast. 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; whilo the blood-vascular ring is a remnant of the 

 primitive blastoccel. Huxley ('Anatomy of Invertebrate,' p. 559) has suggested a similar origin for the nerve-canals 

 (perihamral canals, Ludwig) of the Asterids. 



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

 tentaculiferous grooves by fainter lines. 



