12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



only until a considerable size is reached ; increase in the number of 

 arms is accomplished by breaking off each of the original lo arms 

 at the articulation between the first and second (more rarely between 

 the third and fourth) brachials and forming on the stump an axillary 

 ossicle from which two arms arise, one or both of which may still 

 further divide. 



REGENERATION 



In the crinoids, especially in the feather-stars, the replacement of 

 lost parts by regeneration reaches an miusual extreme. It is most 

 highly developed in the comatulids, pentacrinites and bourgueti- 

 crinites, and least in the Plicatocrinidas and, so far as we know, in 

 Holopus. 



In the pentacrinites it is very common in Metacrinus, nearly as 

 common in Isocrinus, less noticeable in Endoxocrinus, and relatively 

 rare in Coinastrocrinus and in Hypalocrinus. This is not necessarily 

 the result of a greater inherent ability to regenerate on the part of 

 Metacrinus and Isocrinus, but is correlated with the fact that they 

 run up into the shallowest water where the wave action makes itself 

 felt, for the proportion of regenerated individuals decreases rapidly 

 with depth regardless of species. 



Although inhabitants of deep water the species of Ilycrinus and 

 Bathycrinus lose the radials, arms and visceral mass very readily and 

 specimens are frequently found regenerating from the basal ring. 

 Similarly the species of Bythocrinus, Rhizocrinus and Democrinus 

 lose their arms so easily that it is very difficult to secure individuals 

 with the arms still attached to the basiradial cone. 



The family Apiocrinidse as represented in the recent seas is known 

 from two specimens, each representing a different genus. One of 

 these (the type of Proisocrinus ruherrimus) is perfect, the other 

 (the type of Carpenterocrinus mollis) is only a fragment. Similarly 

 the family Phrynocrinidse is known only from two specimens each 

 representing a different genus. One of these (the type of Phryno- 

 crinus nudus) is perfect, though the arms were broken during cap- 

 ture, while the other (the type of N aumachocrinus hazvaiiensis) lacks 

 the arms beyond the radials and the terminal stem plate. 



Among the comatulids regeneration is very common in all littoral 

 species, but with increasing depth it becomes less and less frequent. 

 Broadly speaking it appears to occur to a much greater extent in the 

 Macrophreata than in the Oligophreata, though partially regenerated 

 cirri have been mostly recorded in the latter. It is quite possible that 

 this is connected with the large chambered organ of the Macrophreata ; 

 and it is also possible that it is the small size of the chambered organ 



