DEVELOPMENT OF ANTEDON (COMATULA, LA:\IK.) KOSACEUS. 735 



given off from it on the side opposite to tliat of the first-formed pinnule. Thus tlic 

 formation of the first p;iir of ]nnnules takes place in such a manner that tlie extremity 

 of the arm presents an appearance in the first instance of bifurcation, and tlien of ^r?fur- 

 cation. The new segments henceforth added to the extremities of the arms arc all pin- 

 nated; the pinnules (whose skeleton consists, in this stage, of six or eight simple cylindrical 

 segments) being developed alternately from one side and the other, and being furnished 

 with extensions of the tentaculiferons apparatus of the arms. Owing to the thickness 

 and opacity of the cumulus of condensed sarcode in which they originate, I am not able 

 to speak with positiveness as to the mode of formation of their calcareous skeleton ; but 

 I believe it to take place rather after the manner of the joints of the stem and dorsal 

 cirrhi than after that of the segments of the rays and arms, — that is, to commence with 

 XI complete ring which extends itself longitudinally into a hollow cylinder, rather than 

 by a cribriform plate which wraps itself (so to speak) around the extension of the sar- 

 codic axis (^ G7). It is remarkable that the basal portions of the arms, which had been 

 developed previously to the first appearance of the terminal pinnules, remain destitute 

 of these appendages to the end of the Pentacrinoid stage ; except in the case of the 

 second segment, from which, on each arm, an oral pinnule (^ 54) is developed. — The 

 connexion of the segments of the arms by distinct fibrous tissue is now clearly discernible ; 

 but this tissue corresponds rather with the ligamentous than with the muscular tissue of 

 the adult Antedon. 



7<t. The Stem, in this later stage of Pentacrinoid life, shows no increase in the number 

 of its segments ; but those last formed near its summit are developed to ahnost the same 

 length as the rest ; and all the segments are somewhat augmented in diameter towards 

 their extremities, so as to present somewhat of the dice-box form. The original annulus, 

 which is still distinguishable in the middle of their length, so far from constituting a 

 projection, now lies in a hollow. The axial cavity, if not quite obliterated by the filling 

 up of the segments, is very much contracted ; on this point it is difficult to arrive at a 

 positive determination. The connexion of the segments by a distinct fibrous tissue, 

 resembling that of the arms, and not merely passing from one articular extremity to the 

 other but also embracing the contiguous extremities which it connects, now becomes 

 obvious. — The most important change which the stem presents at this period consists in 

 the enlargement of its highest basin-shaped segment, from which the dorsal cirrhi are 

 developed, and in the further development and multiplication of the cirrhi themselves. 

 This segment, which now presents the aspect in miniature of the centro-dorsal plate of 

 the adult Antedon, augments not only in absolute Ijut in relative diameter, extending 

 itself over the dorsal or outer surface of the basal plates, which, at the time of the detach- 

 ment of the body from the stem, are almost entirely concealed by it. The first-formed 

 whorl of cirrhi now shows itself ready for prehensile action, its terminal claws being 

 hooked, the calcareous segments being bevelled off on their dorsal aspect so as to allow 

 of the downward flexure of the cirrhi, and a considerable amount of contractile fibrous 

 structure being developed between and around the extremities of the segments. A second 



MDCCCLXVI. 5 H 



