152 Mr. E. Billings on the Structure of 



as it is called bj Prof. Thomson, is a small ovate organism, 

 with fom* transverse ciliated bands, a large keyhole-shaped 

 mouth (pseudostome), and a small circular vent (pseudoproct) . 

 These orifices are connected by a rudimentary intestine (pseu- 

 docele). In this stage there is no trace of radiation, and 

 the mouth, therefore, cannot be said to be interradial in its 

 position. 



The nascent Crinoid originates within the jDseudembryo, 

 but developes a mouth, vent, and stomach of its own, all quite 

 distinct from those of its nm-se. This new or permanent 

 mouth is for a short time both oral and anal in its function ; 

 but although in this respect it resembles that of ActinocrinuSy 

 its position in the centre of the ambulacral system shows it to 

 represent the mouth of the adult starfish, while that of Actino- 

 crinus homologizes rather with the oral orifice of the Bijiin- 

 naria. At no time dm'ing its development does the ventral 

 perisome exhibit the structm-e of that of the palseocrinoids, 

 i. e. no orifice in the ambulacral centre, and at the same time 

 one in an interradial space. In the central position of its 

 mouth, and in the possession of an oesophageal ring, Antedon 

 stands above Actinocrinus in rank, and betAveen it and the 

 adult starfish. In none of its stages does it resemble a Biinn- 

 naria either in form or in structm-e. 



9. On some of the Ohjections that have heen advanced against 

 the Vieivs advocated in the jJreceding Notes. 



In all the known species of the existing Echinodermata the 

 mouth is situated in the centre of the ambulacral system ; and 

 it is contended that this fact proves that such must have been 

 its position also in the paleozoic forms. 



This reasoning is not strictly logical. It is true that in the 

 known existing species the mouth is in the centre ; but it does 

 not certainly follow that it is so in all the Echinodermata, 

 living and extinct. Whether it is so or not in any particular 

 fossil species whose structm-e may be under investigation, is 

 a question of fact which can only he positively determined hy 

 direct observation of specimens. On appealing to these we find 

 that, in a large jDroportion of the fossil forms, there is no aper- 

 ture in the perisome at the ambulacral centre. It also becomes 

 evident by the comparison that in general the palajozoic 

 species resemble the embryonic stages of some of the recent 

 Echinoderms, and tliat in these [Biinnnaria for instance) the 

 mouth is interradial. Rules such as that relied on in this case, 

 afford a certain amount of presumptive evidence, which, how- 

 ever, cannot prevail against material and visible facts. When 



