184 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



ramules, and that their present status of pinnules has been due to the rapid increase 

 in the perfection of the arm, extending from the base outward, in the direction of 

 a linear series of supporting ossicles. 



In species of a number of comatulid genera one or more of the proximal pin- 

 nules are normally or occasionally absent, though except for the lack of a pinnule 

 socket the accompanying muscular articulations are of the usual character. This 

 occurs in the genera Comatilia, Petasometra, Gyllojnetra, Decametra, Oligometra, 

 C otylometra, Colobometra^ Prometra, Balanometra, Perometra, Erythrovietra^ 

 Hypalometra^ Atelecrinus, Thaumatocrinus, and Pentmnetrocrinus. Usually the 

 second (first inner) pinnule. Pa, is missing, as in Petasometra^ CyUometra, Deca- 

 metra, Oligometra (very rare), Cotyloinetra (not usual), 

 C'olohometra, Prometra^ Perometra (one species), and 

 Erytlirometra^ sometimes accompanied hy the first (outer) 

 pinnule. P^, as in the outer arms arising from a IIIBr 

 series in Cyllometra, in Balanometra, and in Hypalometra; 

 in Atelccrinvs there are no pinnules on the first 10 or 12 

 brachials: the second, third, and fourth pinnule pairs fail 

 to develop in ComatiHa (fig. 183, p. 98), and Pa, P^. and 

 Ps in ''^ A iitedon " ivipimwta, and in the Pentametrocri- 

 nidae Pj is frequently absent. 



That this is a more or less anomalous state of affairs 

 is shown by the fact that these genera have widely differ- 

 ent affinities, the related forms in all cases having the 

 normal pinnulation, by the more or less constant presence 

 of one or more of the missing pinnules on one or more 

 of the arms, and by the common reappearance of some or 

 all of the absent pinnules on regenerated arms. When 

 pinnules ordinarily absent in these genera are present they 

 are commonly, but by no means always, small and weak 

 and more or less imperfect. 



In most cases it is clear that the absence of the 

 pinnules is the result of the crowding of the arm bases, 

 whereby no room remains for their development. In Comatilia and in '■'■Antedon " 

 impimuita the absence of pinnules seems to be due to arrested development. 



In the Macrophreata great elasticity and a wide range of movement prevail 

 throughout life at all of the muscular articulations and at the synarthries, even 

 though the species may reach an enormous size. The largest species or individuals 

 are but little less pliant than the smallest, or than the young. 



In the Oligophreata, though the young exhibit the same elasticity and range 

 of movement as the young of the Macrophreata, and the smaller species are little, 

 if any, more restricted in their motion, the adults of the larger species as a rule 

 lose a large part of their original power of movement proximally, becoming stiffened 

 and more or less inert. This stiffening of the proximal articulations appears to 

 be correlated with size and number of arms rather than with species, and always 

 begins at the base of the calyx, gradually extending outward during development. 



Fig. 228. — Lateral view oi- 



TYPE SPECIMEN OF CllLOUO 

 METRA OAERETTIANA. 



