314 DR. P. H. CARPENTER ON THE 



angle of the peristome comes to a sudden end on the disk, imme- 

 diately after its first bifurcation ; and all the ambulacral grooves 



Diagram of the disk of a normal Actinometra, with interradial mouth *. 

 The dotted lines mark the interambulacral regions of the disk. A 1 A 2 . . . . 

 Ej E 2 the five pairs of secondary ambulacra. 



of the corresponding ray (C) together with those of the distal 

 arms on the B ray (b x ) are connected with the single groove- 

 trunk, which comes round the rii^ht side of the disk to supply the 

 hinder arms of the D ray (PI. XXYI. fig. G). I cannot account 

 in any way for this abnormal arrangement, which does not appear 

 to be accidental, at any rate so far as the union of the right and 

 left posterior ambulacra is concerned (C 2 and D 1 ). But it is 

 just possible that the parasitic growths which appear upon some 

 of the brachial ambnlaci a may have also shown themselves on 

 the disk, and have destroyed the connection between the pri- 

 mary groove-trunk of the left posterior ray and its peripheral 

 branches, a connection which was not fully restored by the re- 

 generation of that part of the disk. But I imagine that it would 

 he going too far to assume t hat the connection between C 2 and 

 U 1 ambulacra arose in consequence of this loss; for there are so 

 many c;isch in which a large proportion of the hinder arms of an 

 ActinomcJra ;ire entirely devoid of ambulacra, that there would 

 Dot leem to be any absolute necessity for the isolated left poste- 



* Prom flic Report on the ' Challenger ' Comatula), by kind porrniBflion of 

 the Editor, T)r. John Murray. 



