MONOGRAPH OF THE EXISTING CBINOIDS. 583 



In the two following species the pentacrinoids are attached to the pinnules: 

 Ptilometra mulleri,. Thaumatometra nutrix. 



In Ptilometra mulleri the attachment is by means of a circular terminal stem 

 plate; but in Thaumatometra nutrix, where the embryos are retained within a 

 marsupium and there pass through all the larval stages, there is no terminal stem 

 plate, the distal end of the larval column becoming attached to the expanded distal 

 end of a pinnular. 



It is interesting to note that in Ptilometra 'mulleri all, and in Comactinia me- 

 ridionalis most, of the pentacrinoids have been found attached not to fully grown 

 specimens of the same species, but to partially grown individuals, apparently as 

 a result of .successive swarms of larvae drifting away from the original parent mass. 



Cirri. The developmental stage at which the cirri first appear varies con- 

 siderably. 



The first pinnules are formed simultaneously with the brachials which bear 

 them that is to say, the ninth-twelfth. 



The cirri appear before the pinnules in 



Comactinia meridionalis. Antedon bifida. 



Ptilometra mulleri. Antedon mediterranea. 



Crotalometra porrecta. Antedon adriatica. 



Thaumatometra nutrix. 



The cirri appear simultaneously with the pinnules in Antedon petasus. 



The cirri appear after the pinnules in 



fLamprometra protectus. Hathrometra prolixa. 



Heliometra glacialis. Hathrometra tenella. 



Promachocrinus kerguelensis. Hathrometra sarsii. 



Thus in those species in which the column is long and slender and composed 

 of a relatively large number of segments the cirri are later in making their appear- 

 ance than in those species in which the column is short and stout and composed 

 of few segments. 



The first five cirri to appear are interradial in Antedon bifida. 



The first five cirri to be formed are radial in 



Comactinia meiidionalis. Promaichocrinus kerguelensis. 



fLamprometra protectus. Anthometra. ad.riani. 



Ptilometra mulleri. Hathrometra prolixa. 



Crotalometra porrecta. Hathrometra tenella. 



Antedon petasus. . Hathrometra sarsii. 



Heliometra glacialis. Thaumatometra nutrix. 



The interradial appearance of the first cirri in Antedon bifida, attested both by 

 W. B. Carpenter and M. Sars, is difficult to understand, especially in view of their 

 radial appearance in the closely related A. petasus. 



As has already been explained, the first cirri to be formed approach more 

 or less a generalized type, but the later cirri of the pentacrinoids always show all 

 of the essential characteristics of the adults. 



The figures for the number of cirri developed at the time of the casting of 

 the column are unsatisfactory, but it appears to vary between 10 or 12 and 20 

 142140 21 Bull. 82 39 



