MONOGRAPH OF THE EXISTING CEINOIDS. 507 



of these consisting, as before, of a leanike expansion with three tentacles proceeding 

 from its base, of which one is much more extensile than the other two. 



The sarcodic substance which unites the pieces of the skeleton now begins 

 to show a delicate fibrous texture at their adjacent extremities, but no separate 

 fibers are as yet to be distinguished. 



During the later stages of pentacrinoid life the arms continue to increase in 

 length, both by the addition of new segments distally and by the growth of those 

 previously formed, and it is when they have attained the length of about 0.08 

 inch and consist of about 12 segments that we see the first indication of the develop- 

 ment of pinnules. This shows itself at the growing extremity, which now presents 

 a bifurcation, the two rami being in the first instance almost equal, and each tumifiecl 

 at its extremity by an accumulation of sarcode. 



One of these rami grows faster than the other and soon assumes a direction 

 continuous with that of the axis of the arm, from which the other diverges at an 

 acute angle, so that the former comes to be the proper extension of the arm, while 

 the latter takes on the form of a pinnule. Soon, however, the growing point 

 again subdivides, two branches being formed, as previously, and while one of these 

 becomes a continuation of the arm the other is soon to be distinguished as a pinnule 

 given off from it on the side opposite to that of the first formed pinnule. 



Thus the formation of the first pair of pinnules takes place in such a manner 

 that the extremity of the arm presents in the first instance an appearance of bifur- 

 cation, and then of trif urcation. 



The new segments henceforth added to the extremities of the arms are all 

 pinnulate, the pinnules (the skeleton of which consists at 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 tentaculiferous apparatus of the 

 arms. 



Owing to the thickness and opacity of the cumulus of condensed sarcode in 

 which they originate, Carpenter was not able to speak with positiveness as to the 

 mode of formation of their calcareous skeleton, but he believes it to take place 

 rather after the manner of the ossicles of the column and dorsal cirri than after 

 that of the segments of the division series and arms; that is, to commence with a 

 complete ring which extends itself into a hollow cylinder rather than by a cribri- 

 form plate which wraps itself, so to speak, around the extension of the sarcodic 

 axis. It is remarkable that the basal portions of the arms, which had been de- 

 veloped previous 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 is developed. 



The connection of the brachials by distinct fibrous tissue is now clearly dis- 

 cernible, but this tissue corresponds rather with the ligamentous than with the 

 muscular tissue of the adult Antedon. 



At the beginning of the free stage the segments which form the skeleton of 

 the free arms depart even less than the primibrachs from the simple type of con- 

 formation which they present at their first development, being still obviously 



