294 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



thoso found in the other <i;enera of the Thalassomctriilse, there is, curiously enough, 

 never the slightest trace of any modification of the proximal pinnules toward the 

 type found in the other genera of that group. 



Dr. W. B. Caipenter has noticed in the growing young of Antedon hifida tliat, 

 as in other species, after the formation of the firet two whorls of cirri no special 

 regularity can be traced in the manner of development; the young cirri normally 

 appear between those previously formed and the radial pentagon, so that tlioir 

 sockets are close to the margin of the centrodorsal; but as the centrodorsal grows 

 and new cirri appear around its margin, the older cirri which are attached close to 

 the doi-sal pole drop away and their sockets become gradual!}' obUterated by cal- 

 careous deposit. The result is that the doreal surface of the centrodorsal is usually 

 left comparatively smooth, but in some species the deposit of new material con- 

 tinues after the cirnis sockets are obliterated and causes the dorsal pole to become 

 rough and irregular. On the other hand, the lower surface of the centrodoreal in 

 most species of the Comasteridae is almost flat and extremely smooth. This is 

 o^ving to the very extensive and uniform manner in which the new material is 

 laid down. 



Dr. P. H. Cai-penter noticed that the primary trunks which leave the chambered 

 organ, subsequently dividing and passing to the cirri in the corresponding radial 

 areas, usually undergo their division within the cavity of the centrodorsal. It 

 sometimes happens, however, that more or less of this division takes place within 

 the substance of the centrodorsal, so that interiorly there may be onl\^ one radial 

 opening visible, whereas outwardly there may be found the apertures of half a dozen 

 cirrus canals. 



In regenerating cirri the basal segments are the longest, and the following 

 decrease rapidly in diameter, so that the whole cirrus tapers considerably from its 

 base to its point. This condition gradually becomes less and less marked as the 

 segments increase in size and their apposed faces become beveled ofi toward the 

 dorsal side, so that the cirrus ultimately acquires all the characters of maturity. 



In the comatulids only the first few rows of cirri are developed, as described by 

 W. B. Cai-penter. The cirri wliich appear subsequently gradually assume certain 

 of the developmental features of regenerating cirri, so that at the adult stage, and 

 usually some time before that stage is reached, the cirri wliich are constantly pro- 

 duced about the ventral margin of the centrodorsal arise exactly as if they were 

 formed at an old socket from which the original cirius had been lost. 



In very old specimens of certain species a peculiar condition is found among 

 these last formed marginal cirri, which was firet noticed in Floromctra ma^eUanica. 

 The cirri are formed just as regenerating cirri, but with increasing age the ontogeny 

 of regenerated parts becomes gradually retarded, so that in old examples the last 

 formed cirri never assume mature charactere, but remain slender and tapermg. 



As the assumption of a definite number of segments and the cessation of further 

 addition after the full number is reached is a ti-ue and definite growth character and 

 therefore dependent, like aU other growth charactci-s, upon the viriUty of the animal, 

 incipient senescence affects this likewise, and the marginal cirri of very old specimens 

 therefore possesses the number of segments characteristic of the adult, plus an 



