MONOGRAPH OF THE EXISTING CRINOIDS. 



413 



of the disk was known. The first notice of a species of Zygometra, also published 

 in 1868, was based upon a detached disk, which was described as a recent cystid, 

 while the type species was called microdiscus on account of the original specimen 

 possessing a disk only half grown. 



In types in which the arm bases make a relatively small angle with the 

 dorsoventral axis, and therefore inclose the visceral mass on all sides except the 

 ventral, as in the Thalassometridae, Charitometridae, and Atelecrinidae, loss of 

 the disk rarely, if ever, occurs, and no cases of regeneration of the disk have 

 been recorded. It appears to be extremely rare in the remaining families. 



Spontaneous evisceration has never been proved in any species. 



Individuals with one, or even two, radials regenerated have been noted in 

 Ptilometra miilleri, Antedon bifida, and A. mediterranea, and in a few other 

 species. Loss of a radial can only come about through a serious accident. 



Eegeneration of the cirri occurs throughout the comatulids but is not so 

 common as the regeneration of the arms and pinnules. In the macrophreate types 

 the cirri, which are very fragile and often deciduous, are usually (but not always) 

 lost in their entiety, and total regeneration supervenes. In the oligophreate 

 types, in which the whole animal is usually less fragile than in the Macrophreata, 

 the cirri are commonly broken off at some distance from the base, and the distal 

 portion regenerates, though loss of the entire cirrus and total regeneration also 

 occurs. Partial loss of a cirrus is common in the species of Tropio?netra, and is 

 not rare in the species of Charitometridse and of Ptilometra. Elsewhere it is 

 of more or less casual occurrence. In certain species of Thalassometridae with 

 long conical centrodorsals the earlier cirri are cast off and replaced by short, 

 irregular, rounded-conical spines with their tips crowned with spinelets. 



In total regeneration of the cirri, as described by Minckert and seen in species 

 of Crinometra, a small peg-like process appears on the cirrus socket in the central 

 canal. As this increases in size segmentation makes its appearance, and it becomes 

 divided into a number of narrow rings which in the proximal portion soon become 

 short cylinders though remaining ringlike distally where the new segments are 

 formed. 



Regarding Antedon bifida at Roscoff, Perrier writes that nothing is commoner 

 than to find arms which are in process of regeneration. The animals do not break 

 up spontaneously, and they are not even as fragile as many ophiurans of much 

 larger size ; but the movements of the waves, their habit of entangling themselves 

 with each other and with the branches of the seaweeds among which they live, 

 as well as the actions of the animals living in the same habitat, cause a considerable 

 amount of mutilation. 



The mutilation is not of long duration, for the lost parts very soon regenerate. 

 At the end of a few days a white digitation appears on the stump, and the lost 

 member, pinnule or arm, soon is replaced. 



On August 9 Perrier removed the extremity of an arm of a carmine Antedon 

 which had been kept in a basin for eight hours. Near it was a vermilion Antedon 

 with all the arms but three cut off more or less near the base and in process of 



