118 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



place in the brachials and pinnulars is a shortening and broadening, combined 

 with the gradual appearance of the adult ornamentation if any be present. 



The young of the multibrachiate comatulids are also 10-armed until a con- 

 siderable size is reached, structurally agreeing in every way with the young of 

 the permanently 10-armed forms. 



I have examined the 10-armed young of Capillaster multiradiata, Comanthus 

 pinguis, C . japonica, C. parvicirra, Himerometra martensi, and Stephanometra 

 monacantha, and of numerous other species belonging to the Comasteridse, Zygo- 

 metrida;, Himerometridae, Mariametridse, Colobometridse, Calometridne, Thalasso- 

 metridse, and Charitometridse. 



In the case of the 10-armed young of the species of Himerometra generic 

 determination is very easy, as the characteristic enlarged proximal pinnules are 

 conspicuous from the first; and the same is true of the 10-armed young of the 

 species of Stephanometra, in which the distinctive spiniform proximal pinnules 

 appear at a very early age; but in the great majority of the multibrachiate species 

 specific determination of the young is in general only possible by inference, based 

 upon the association of the young with the adults, since the structure of the free 

 undivided arms and of the pinnules which they bear varies but little within broad 

 groups, and the division series upon which we depend for the determination of 

 the adults are lacking. Furthermore, as in the proximal pinnules, which do 

 not appear until several of the outer pinnules have been formed, Pj appears 

 before P 2 , and P 2 appears before P 3 , the characteristic enlargement of P 2 in such 

 types as Lamprometra and of P 3 in various species of Heterometra and Dichro- 

 metra is naturally relatively late in becoming evident, for P t attains a considerable 

 growth before P 2 and P s are formed, and it is some time before they overtake it. 



The postradial series of these 10-armed young of multibrachiate types in 

 every way resemble those of the adults, except that all the division series lying 

 beyond the IBr series are lacking, and the free undivided arms, 10 in number 

 and strictly comparable with the free undivided arms of the adults, arise directly 

 from the IBr axillaries instead of being separated from them by a greater or 

 lesser number of ossicles. 



Interpolated division series. 



Since the free undivided arms of the 10-armed young are identical in structure 

 with those of the adults, but arise directly from the IBr axillaries without the 

 intervention of division series, it is evident that the postradial structure of the 

 adults differs from that of the young in the interpolation of numerous ossicles 

 in the form of division series between the IBr axillaries and the bases of the 

 original 10 arms, together with a reduplication in the number of the arms (see figs. 

 140, 142, 144, 146, 149, 151, 153. 155, 157, 159, p. 83; 166, 167, 168, 170, 172, p. 86; 

 174, p. 89; 1015, 1016, pi. 6; 1018, pi. 7; 1019, 1020, pi. 8; 1021-1023, pi. 9; 1024- 

 1026. pi. 10; and 1029, pi. 11). 



