418 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



other forms in the remaining genera, the disk is sometimes lost, though, speaking 

 generally, in this family loss of the disk is very unusual. 



In the species of Stephanometridse individuals in which the disk has been 

 lost are not uncommon. 



The species of Colobometridse all lose their disks rather easily, especially the 

 10-armed species and the species of Cenometra. In one of the species of this 

 genus which is known from only two specimens, C. emendatrix, the disk has not 

 been observed except in the early stages of regeneration. 



The Tropiometridce are extraordinarily tough, and in this family loss of the 

 disk, as well as loss of any other part of the body, is rare. 



In the Calometridae, especially in the genus Neometra, the attachment of the 

 visceral mass is peculiarly weak, it being more frequently lost even than in the 

 Zygometridae. 



Among the species of Thalassometridse and Charitometridse loss of the disk 

 is as a rule very exceptional, since in these families, as a result of the compression 

 of the division series and arm bases, it is entirely inclosed both dorsally and 

 laterally. The only genus in which the disk is frequently lost is Strotometra, 

 which is characterized by the absence of the protective approximation of the 

 division series. 



Loss of the disk is common in the subfamily Antedoninae, in which the division 

 series are narrow and in life are extended almost at right angles to the dorso- 

 ventral axis ; but in the other macrophreate types it is very rare, and quite unknown 

 in the Atelecrinidse and Pentametrocrinidse. 



As a general rule, it may be stated that comatulids having well separated 

 rays, a convex disk, and a patent habit lose their visceral mass readily, especially 

 if it be plated; but those with the rays and division series in close contact or 

 with the ventral surface of the disk normally flat or more or less concave lose 

 their visceral mass seldom, or not at all. 



Prof. Arthur Dendy has made a special study of the regeneration of the 

 visceral mass in Antedon bifida, his observations having been carried out at Mill- 

 port on the Clyde. 



It was first noticed in 1592 by Fabius Columna that the visceral mass of 

 Antedon mediterranea was readily separated from the rest of the animal. Dr. 

 W. B. Carpenter remarked the same thing in Antedon biftda at Oban, while the 

 naturalists of the Challenger expedition often noticed it in various other forms. 

 Prof. A. Milnes Marshall had observed its frequent occurrence in Antedon medi- 

 terranea at Naples, and had found that it was easily brought about by artificial 

 means. He also observed that after evisceration regeneration promptly began, 

 and he collected a series of specimens for the further study of the process which 

 he turned over to Professor Dendy, and it is upon these that part of the latter's 

 studies are based. 



Dendy noticed that the concave surface of the calyx is lined with a thin 

 layer of connective tissue, on which the visceral mass rests and to which it is very 

 closely attached. 



