88 THE YOYAGE OF H.M.S. CHALLENGER. 



VI.— THE MINUTE ANATOMY OF THE DISK AND ARMS. 



I do not propose to treat the subject of this chapter as fully as I have clone the 

 comparative morphology of the Crinoid skeleton. Much has been written about it lately, 

 and a general resume of this recent work, together with some independent observations 

 of my own, was published in the Quart. Journ. Micr. Sci. for April 1881. 



Since that time I have been able largely to increase the range of my observations on 

 the anatomy of the Comatulae, and have also extended them to Pentacrinus, Bathycrinus, 

 and Rhizocrinus. The result has been that I am able to confirm in almost every respect 

 the admirable investigations of Ludwig on the minute anatomy of Antedon rosacea.^ 



On the other hand, there are some points, notably in the relations of the axial cords 

 of the skeleton, which were entii'ely overlooked by him; while he also omitted to describe 

 some remarkable peculiarities in the structure of the plexus of blood-vessels which is 

 situated in the lip. and is connected with the oral blood-vascular ring. 



It is only fair to state, however, that my new observations upon the nervous and 

 vascular systems of this type owe their origin, in great measure, to my' having been aljle 

 to examine other species in which the peculiarities in question are much more developed 

 than thev are in Antedon rosacea. 



A. The Geography of the Disk. 



I find that it is most convenient, on the whole, to use the terms right and lift 

 precisely as in human anatomy. When the ventral surface is upwards, with the mouth 

 north, or pointing away from the observer, and the anus posterior, the right side of the 

 disk would be west in a map, and the left side east (figs. 2, 3). On the other hand, 

 when the dorsal surface of the skeleton is upwards, the anal area being, of course, 

 posterior, the eastern rays are those of the right side, and the left rays are in the west. 

 The same m^ethod applies to the arms, a. pinnule on the right side being east in a dorsal 

 view, and west in a ventral one. 



B. The Digestive Tube. 



Little need be said about the alimentary canal, the general course of which alters 

 but slightl}^ in the endocyclic Crinoids {Antedon, Pentacnnus, Rhizocrinus, &c.), though 

 it varies a good deal in the complexity of its cavity. Both in Antedon rosacea and in 

 Antedon eschrichti the gullet runs downwards and backwards, trending slightly to the 



1 Op. cit., Zeitschr.f. uiss. Zool. 1877, Bd. xxviii. pp. 255-353, Taf. xii.-xix. 



