l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



THE COURSE TAKEN BY SPECIALIZATION AMONG THE 



CRINOIDS 



The dominant feature of the progressive speciaHzation among the 

 crinoids from the earhest times to the present day has always been 

 a process of progressive simphfication in structure, the result of a 

 process of progressive atrophy or suppression affecting some part or 

 other of the organism. Thus the more specialized types differ from 

 the more generalized through the atrophy or suppression of some 

 important structural element, while the later groups are differentiated 

 among themselves according to the lines which this atrophy or sup- 

 pression has followed. 



The (recent) Articulata are distinguished from the Inadunata by 

 the sudden cessation of stem growth (with an apparent, though not 

 real, exception in the pentacrinites) after the stem has attained a 

 definite and fixed length, and by the extreme atrophy of the calyx 

 involving in most cases the complete disappearance of certain essential 

 elements ; the comatulids are differentiated from all the other (recent) 

 types by the suppression of the column excepting for the proximal 

 or topmost columnal which becomes permanently attached to the 

 calyx; Holopus is differentiated from all other (recent) genera by 

 the suppression of the column, the infrabasals and the basals, the 

 stalk being formed by the coalesced and elongated radials ; the 

 Phrynocrinidse diifer from the Bourgueticrinidas in the complete sup- 

 pression of the radicular cirri, and the Bourgueticrinidse diifer from 

 the Phrynocrinidse in the suppression of the terminal stem plate. 



THE OCCURRENCE OF LITTORAL CRINOIDS 

 Except on sandy and exposed muddy shores littoral crinoids occur 

 in all possible situations. Their one essential requirement is pure, 

 well-aerated water having a relatively high minimum salt content and 

 well provided with minute plankton organisms, and wherever this 

 condition is met within the range of the littoral species they may be 

 looked for in the water just below the low-tide mark or in protected 

 situations ; sometimes they even occur in regions left bare at low tide. 

 Along the shores of the Indian Ocean from southeastern Africa, 

 Madagascar and Mauritius to Suez, India and the Malay archipelago, 

 along the coasts of Australia, especially in the north, and thence north- 

 ward to Fokien and southern Japan, littoral comatulids of many 

 species are abundant — about 30 are known from Singapore alone — 

 particularly on reefs and rocky shores, less commonly in sheltered 

 situations and in eelgrass, though their occurrence is commonly more 

 or less local and they are frequently not to be found in apparently 



