MONOGRAPH OF THE EXISTING CKINOIDS. 169 



In all of the genera of the Comasteridse, excepting those in which syzygies 

 occur in place of one or more of the synarthries, the arms and cirri are unusually 

 tough, so that almost or quite perfect specimens are rehitively easy to secure : even 

 the minute Comatilia from great depths is but rarely mutilated, and the same is 

 true of the slender and delicate Leptonemaster. 



As in the comasterid genera Comaster, Vomatetla, and N eocotnatella^ the species 

 of all the genera of Zygometridae in whicli the disk is small are especially liable to 

 fracture of the arms at the most proximal syzygy, which in these genera replaces 

 the synarthry between the elements of the IBr series; but the species with large 

 disks are not so brittle. In this family, and in tlie Himerometridse and Colobo- 

 metridaj, the tendency of the arms to break at the first brachial syzygy as a rule 

 does not appear to be so strong as in the MariametricUe and Stephanometridse. 



It is somewhat difRcidt satisfactorily to compare the species of the Thalasso- 

 raetridse with those of the preceding families because of the much rougher treat- 

 ment necessary in capturing them, owing to the ordinarily greater depth of their 

 habitat, where they are usually found among stones and coral. Generally speak- 

 ing, they seem to be peculiarly tough, as tough as the species of Comasteridas with, 

 as in that family, a few curious exceptions. The species of Ptilometrinoe, with 

 the single exception of the species of Tropiometridse, are perhaps the least brittle 

 of any of the comatulids, and it is astonishing to see how the individuals will stand 

 the roughest sort of treatment in the dredge and yet remain whole; on the other 

 hand, the species of Stenometra, Daidalonietra, and Stiremetra are extremely brittle, 

 and it is almost impossible to obtain anything like satisfactory specimens of any 

 species of either of these two genera. All of the other genera of the Thalasso- 

 metrina? are tough, though this is not quite so marked as in the Ptilometrinse. 



The species of Calometridte and Charitometridse are all very tough, on the 

 whole resembling those of the Ptilometrinie and Tropiometridae. 



As a result of the toughness of the species of these groups the cirri are some- 

 times torn apart at a greater or lesser distance from the base and the outer portion 

 regenerated. I have observed such partial regeneration of the cirri in Ptilometra^ 

 Pterometra, Asterometra^ Trofiometra (part 1, fig. 319, p. 275), ('nnometra^ 

 Pachylovietra, and Glyptometra , and in some of the genera of the Calometridse. 

 Outside of these genera it has only been reported in Promachocrimts, and as a result 

 of artificial mutilation in Antedon. 



In the Macrophreata there is a very considerable diversity in the readiness with 

 which tlie arms and cirri are lost, but the species as a whole are far more brittle 

 than the species of Oligophreata. A curious diiference between the two is that 

 whereas in the Oligophreata bi-achial fracture usually occurs at the first syzygj', in 

 the Macrophreata it quite as frequently, or even more commonly, occurs at the sec- 

 ond or third syzygy, or even farther out on the arm. 



The toughest of all the forms in the ]\Iacrophreata are the species of the genera 

 of Perometrinse and Antedonina?, which in this respect, as in others, approach the 

 species of the Oligophreata. All of the genera in these two subfamilies are about 

 equally tough. 



