MONOGRAPH OF THE EXISTING CRINOIDS. 525 



The column of the pentacrinoids of H. sarsii is composed of usually about 

 40 (in some cases as many as 44, or even 49) segments. In A. bifida the number 

 varies between 16 and 27, averaging about 20. 



The terminal stem plate in H. sarsii is provided with long digitiform pro- 

 longations. In A. bifida it is circular or lobate and only very rarely shows short 

 and broad processes. Sars suggests that the characteristics of the terminal stem 

 plate of H. sarsii are necessitated by the large size of the larvae. 



At the time of detachment from the larval column H. sarsii is much farther 

 advanced than A. bifida. There is a much greater number of pinnules, which 

 occur all along the arm, and the centrodorsal bears a very much larger number 

 of cirri. 



H. sarsii breaks away from the larval column from the end of March to May, 

 while A. bifida becomes detached between the middle of August and the middle 

 of September. 



Sars' youngest pentacrinoids were taken at the beginning of July and his 

 oldest in March, April, and May, together with young free living individuals 

 which evidently had just become detached, and which were no larger nor in any 

 way more developed than the oldest of the pentacrinoids. Some of them were 

 even less developed than the latter. 



Sars found eggs in the pinnules of H. sarsii in June. Assuming that the sexual 

 products are developed in the early summer and at no other season (which, however, 

 is not established), it would seem that the development of the pentacrinoids of this 

 species takes place much more slowly than that of the pentacrinoids of Antedon, 

 occupying almost an entire year. 



Regarding the pentacrinoids of Antedon bifida, J. V. Thompson says that they 

 possess the power of bending or inclining the column freely in every direction, 

 and, what is more remarkable, of twisting it up into a short spiral, and that 

 with a considerable degree of vivacity a kind of movement that has not been 

 noticed except in Vorticella. He speaks of the arms as at one time spreading 

 outward like the petals of a flower, at another rolled inward like an expanding 

 bud. 



Allman said that his specimen, in the " prebrachial stage," was very irritable, 

 and on the slightest annoyance the tentacles were suddenly withdrawn and the 

 valvelike plates of the pyramidal roof instantly closed down over them. In 

 this retracted state it might remain for a long time, wearying the patience of 

 the observer, who might have to wait for hours together before he could again 

 obtain a satisfactory display of its structure. 



J. V. Thompson stated that from observations repeatedly made he thought 

 it most probable that the comatulids attain their full growth in one year, so 

 as to be in a condition to propagate their kind the summer following that of their 

 birth. 



At that time (viz.. May and June) these full-grown individuals have the membraneous expan- 

 sion inside each of the pinnae considerably extended, at least as far as the fifteenth or 

 twentieth pair. These, which are the matrices or conceptacula, at length show themselves 

 distended with the ova which in July, and e v en earlier, make their exit through a round 



