MK. P. 11. CARPENTER ON THE GENUS ACTINOMETKA. 445 



arm aud pinnules becomes convex, and does not show the least 

 trace of a groove of any description. 



The position of the point at which the two folds of perisom 

 bounding the original ambulacral groove meet and unite, varies 

 extremely : the union may, though rarely, take place on the disk ; 

 sometimes it is at the base of the arms, and sometimes not till near 

 their middle or terminal portions. In any case, however, the fusion 

 where it occurs is so complete that all trace of the original amlu- 

 lacral groove is entirely obliterated. 



This fact has a very important bearing upon the two opposite 

 views which have been recently advanced regarding the nervous 

 system of Comatula. 



According to the one view, held by Dr. Carpenter and by the 

 author, the axial cords traversing the centre of the calcareous seg- 

 ments of the skeleton, together with the fibrillar envelope of the 

 quinquelocular organ from which they all radiate, constitute the 

 principal and, as the author is disposed to think, the only nervous 

 S3'' stem of Comatula. 



In the centre of every segment of the skeleton, from the first 

 radials to the tips of the arms and pinnules, and also in the cirrus- 

 segments, these axial cords increase considerably in size and give 

 off four main branches. Two run towards the ventral side and in 

 the calyx disappear in the neighbourhood of the muscles connecting 

 the segments ; but in the arms they continue their course towards 

 the ventral perisom and break up into numerous small branches, 

 some of which may be traced as far as the tips of the crescentie 

 leaves. The two inferior or dorsal trunks run towards the dorsal 

 surface of the skeleton ; and while some of their branches are lost 

 in the plexus of tissue forming its organic basis, others seem to 

 become connected with epidermic structures. In the arms of 

 Antedon celtica Dr. Carpenter has found branches of the axial 

 cords ramifying upon the ends of the muscular bundles connect- 

 ing the segments ; and the results of his experiments have shown 

 that the power of motion in any arm depends upon the connexion 

 between its axial cord and the fibrillar envelope of the chambered 

 organ,while the power of coordinating the regular swimming move- 

 ments of all the arms depends upon the integrity of this envelope, 

 which there is thus good reason to believe to be of a nervous nature. 



The chief objection to this view is that it places the nervous 

 system of the Crinoids on the dorsal side of the body, and not on 

 the ventral side immediately superior to the water-vessel as in 



