152 Mr. E. Billings on the Structure of 



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

 with four ti-ansverse ciliated bands, a large keyhole-shaped 

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

 These orihces 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 iuterradial in its 

 position. 



The nascent Crinoid originates within the pseudembryo, 

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

 distinct from those of its nurse. 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 Actinocrinus, 

 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 Bipin- 

 naria. At no time during its development does the ventral 

 perisome exhibit the structure of that of the palajocrinoids, 

 {. 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 between it and the 

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

 naria either in form or in structure. 



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

 the Views advocated in the preceding 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 palaeozoic 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 structure may be under investigation, is 

 a question of fact which can only he jjositivelg determined hg 

 direct observation of specimens. On appealing to these we find 

 that, in a large proportion 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 palaeozoic 

 species resemble the embryonic stages of some of the recent 

 Echinoderms, and that in these [Bijnnnaria for instance) the 

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

 affoi'd a certain amount of presum})tive evidence, which, how- 

 ever, cannot prevail against material and visible facts. When 



