NOTES ON EOHTNODERM MORPHOLOGY. 611 



fact that any one fails to demonstrate the existence of a certain 

 anatomical structure is no proof of the non-existence of that 

 structure. Two instances already mentioned, viz. the connec- 

 tion of the ovoid gland in the Urchins with an oral ring, and 

 the existence of both coeliac and subtentacular canals in the 

 arms of Crinoids are cases in point. Another, which will be 

 noticed immediately in more detail, is that Professor Perrier, 

 working with young and fresh material, has seen the connec- 

 tion between branches from the axial cords of the arms and the 

 muscle-fibres, which I have long sought for in vain in spirit 

 specimens. 



Both Ludvvig and myself have found that certain anatomical 

 points are not easily demonstrated in Ant. rosacea, whereas 

 they are much more evident in the larger Ant. eschrichti. 

 I have five series of sections through the disc of this species ; 

 and have also cut four other species of Ante don, four of 

 Actinometra, and three of Peutacrinus, together with 

 Promachocrinus, Rhizocrinus, and Bathycrinus. I 

 venture to think, therefore, that I am on the whole in a better 

 position than Prof. Perrier for forming a judgment respecting 

 the anatomical relations of the central plexus. In one respect, 

 however, my opportunities have been inferior to his. All my 

 work has been done on material which has been some time in 

 spirit; and though this affects anatomical structure but little, 

 it makes a vast amount of difference in histological work. 

 Prof. Perrier, on the other, hand has had access to an abundant 

 supply of fresh Ant. rosacea of all ages; and his statement 

 that the histological structure of the " ovoid gland " of this 

 type is identical with that of the same organ in other Echino- 

 derms ^ must therefore be received as authoritative. But 

 when ho says that the intervisceral blood-vessels described by 

 Ludwig and myself as originating in this organ are merely 

 ramifications of the gland ending in apparently blind dilata- 

 tions, I must totally disagree with him. That the walls of the 

 central plexus are of a glandular nature must be apparent to 

 everyone who has examined a section of it. But I have also 

 • As described by himself, Aposl.olides, or Koclilor? 



