252 



ON THE PRE-BllACHIAL COMATULA. 



I do not think that the little animal which forms the subject of the present 

 paper will throw much light on the nature of the Cystidea, though in some 

 respects it reminds us of them. It is true that its long tentacula-like appendages 

 forcibly recall by their position the arms, more or less strong indications of which 

 have been shown by the observations of Volborth, Ed. Forbes, and J. Muller, 

 to exist in probably all Cystidea. The long cylindrical appendages described by 

 Volborth in Echino-encrinus angulosus, and E. striatus, and by Forbes in 

 Prunocystites, will more especially occur to us in this comparison. No homological 

 identity, however, can be admitted between the two sets of organs ; for while the 

 arms of the Cystidea are referable to the same category as those of the ordi- 

 nary crinoids, and have a well-developed calcareous skeleton, the cirri of the 

 young Comatida belong to the ambulacral system. The chief point of resem- 

 blance lies in an apparent identity of position ; but it will be remembered that 

 an essential character in the Cystidea is the reduction of the ambulacral zone to a 

 minimum, with the expansion of the antambulacral zone to a maximum. Now, 

 just as in the ordinary Crinoids, the arms of the Cystidea are situated upon the 

 boundary between these two zones, and their origins are therefore, by the 

 reduction of the ambulacral zone, necessarily carried inwards towards the vertex ; 

 while in the pre-brachial larva of Comatida, the ambulacral zone is well de- 

 veloped, and the cirri arise not from its peripheral boundary, but within this 

 boundary, from an area immediately surrounding the mouth. 



Description of Plate XIII. 



Pre-Brachial Comatula greatly enlarged. 



Fig. 1. The animal with its roof-plates fully expanded, and the cirri extended from between their 

 edges. 



2. Outline of the same animal, in the act of expansion. 



3. Outline of the same with the cirri entirely withdrawn, and the roof-plates closed, 



4. The extremity of a cirrus still more magnified. 



5. Outline of the body looking down upon it from the vertex. 



