82 CATALOGUE OF THE BLASTOIDEA. 



the mouth would have effectually prevented the passage of immature ova along the 

 ambulacra in the opposite direction, as suggested by White. Knowing that the ova 

 of a Crinoid are developed in the pinnules, and believing these appendages to be 

 represented by the so-called pinnules of a Blastoid, he was driven to suppose that 

 although the ovaries of a Blastoid might be within the body, the ova after germinating 

 found their way to the bases of the pinnules, where they could be developed and dis- 

 charged. In the Crinoids, however, the actual ovaries are situated in the pinnules 

 and are much more than the little accessory sacs described by White. 



White's views have found favour with no one but Billings l , who described him- 

 self as being in perfect agreement with them. " The central aperture is not the 

 mouth ; in fact it is not a natural orifice, but a breach in the summit caused by the 

 destruction of a portion of the vault ; " and he went on to say that the true natural 

 orifices are the minute pores at the edge of the covered peristome where the ambu- 

 lacra pass under the vault. It may be noted, however, that these pores do not exist in 

 specimens which have the covering plates preserved on the ambulacra (PL I. fig. 8). 

 Billings believed the median grooves of the ambulacra to have been exclusively 

 occupied by the ovarian tubes, so that "then the arm of Pentremites would have the 

 respiratory portion of the ambulacral system on its dorsal, and the ovarian portion on 

 its ventral aspect. In the true Crinoids, both the respiratory and ovarian tubes are 

 situated in the groove in the ventral side of the arm 2 ." 



We have pointed out above, however, that the hydrospires can in no way be 

 regarded as a respiratory portion of the ambulacral system ; while the analogy of 

 recent Crinoids goes to show that the ambulacral groove of the Blastoids was a 

 ciliated food-groove, and that it was not occupied by any portion of the generative 

 apparatus, as supposed by Billings, whose theories as to the position of the mouth 

 and generative apparatus in the Palseocrinoids and Blastoids were the result of a 

 total misapprehension of the anatomy and physiology of recent Crinoids. He often 

 confounded the arm-groove on the ventral surface of the skeleton with the ambu- 

 lacral groove on the ventral surface of the arm, which is separated from the arm- 

 groove by the nervous, vascular, and generative trunks, and by two radiating 

 extensions of the body cavity. Much of Billings's reasoning upon the Palaeocrinoids 

 and Blastoids depends on the erroneous supposition that all these organs were 

 situated in the ambulacral groove, whereas in reality they are all below it. Had he 

 properly understood this fact, he could never have supposed that the ambulacral 

 groove of a Blastoid is so completely filled up by an ovarian tube that almost no 

 room remained for currents of water conveying food to the central opening, which 

 he refused to regard as a mouth on account of its being closed by a vault. 



1 American Journ. Sci. 1869, vol. xlviii. p. 83; Ann. & Mag. Nat. Hist. 1870, vol. v. p. 265. 



2 Ibid. 187ii, vol. 1. p. 227; id. 1871, vol. vii. p. 144. 



