120 



Jour. N, H., vol. vii, pp. 486,487). The main difference between the 

 Cjstidea and the Blastoidea is, that in the former the hydrospires do not 

 communicate with the pinnules, whilst in the latter the cavities of the pin- 

 nulse and hydrospires are directly connected by the ambulacral pores. 



The development of the recent Crinoid Antedon rosaceus, as described 

 by Prof. Wyville Thomson (Phil. Trans., 1866), persues a course that 

 could not possibly result in the production of such an animal as Actino- 

 crintts. The pseudembryo, as it is called by Prof. Thomson, is a small 

 ovate organism, with four transverse ciHated bands, a large key-hole-shaped 

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

 orifices are connected by a rudimetary intestine (pseudocele). 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 orginates 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 Actmo- 

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

 represent the mouth of the adult Star-fish, while that of Actinocrimts 

 rather homologates with the oral orifice of the Bipinnaria. At no time 

 during its development does the ventral perisome exhibit the structure of 

 that of the paloeocrinoids, 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, Aritedon stands above 

 Actmocrinns in rank, and between it and the adult Star-fish. In none 

 of its stages does it resemble a Bipinnaria either in form or in structure. 



9. On some of the objections 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 reasoming 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 be so or not in any particular fossil species whose structure may be 

 under investigation, is a question of fact ivhich can only be positively 

 determined by direct observation of specimens. On appealing to these we 

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



