ME. P. H. CARPENTER ON THE GENUS ACTINOMETRA. 449 



Still less do Teusclier and Lange agree about tlie nervous system 

 of the Ophiurids. Lange's ganglionic masses are described as 

 artificial by Teuscber, who, as in the cage of the Asterids, regards 

 as the nerve only the fibrillar structure representing the subepi- 

 thelial band of Comatula. 



The question is still an open one : and it is therefore of no 

 small interest to find that this subepithelial band, the so called 

 ambulacra! nerve, is not always present in the arms of Comatula, 

 and that even, when it exists it is certainly not motor in 

 function. 



It has been stated above that in certain of the arras of Acti- 

 nometra the water-vessels are simple tubes like the integumentary 

 water-vessels of the Molpadidae, and not in connexion with any 

 tentacular apparatus. Whether the mouth be radial or interradial, 

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

 latter case they belong to the trivium, as in A. polymorpha, and 

 in the former to the bivium, as in A. Solaris. 



In only one out of twelve specimens of A. polymorpha has the 

 author found a non-tentaculiferous arm in the bivium : it was in one 

 of the two anterior radii. But this specimen was very remarkable ; 

 for out of 31 arms 19 were entirely devoid of a tentacular appa- 

 ratus, and in 15 of these the union of the two sides of the ambu- 

 lacral grooves had taken place either on the disk or in the basal 

 ai'm-segments ; so that an " ambulacral nerve " was wanting in 

 nearly half the total number of arms. In the other four of these 

 non-tentaculiferous arms the groove remained open for a short 

 distance, and then closed in the manner already described. Tliree 

 of these four arms belonged to the trivium ; but the fourth was 

 an anterior arm belonging to the bivium, and was borne upon the 

 same palmar axillary as a well-developed ordinary tentaculiferous 

 arm. 



With this exception, the author has invariably found the non- 

 tentaculiferous arms on the aboral side of the disk: their number 

 and distribution, however, vary extremely, not only in difi"erent 

 species, but in different individuals of the same species. Thus in 

 A. poJymorpha the author has found the proportion of non-tenta- 

 culiferous arms to the total number to vary from -^-^ to ^^. Even 

 in two individuals with the same number of arms it may not be 

 the same: thus in two specimens with 20 arms the proportion 

 was -2^^ and -|^ respectively, and again ^ and \^ ; while in one 

 specimen all the arms were normal and tentaculiferous as in 

 Antedon. 



