1870.] BILLINGS — ON CRINOIDEA AND BLASTOIDEA. 191 



true Cj'stidean. Its affinity to Cadaster was first pointed out by 

 Dr. C. A. White, who also suggested that it should be assigned 

 to a distinct group. (Bost. Jour, N. II., vol. vii, pp. 486487). 

 The main difference between the Cystidea and the Blastoidea is, 

 that in the former the hydrospires do not communicate with the 

 piunula3, whilst in the latter the cavities of the pinnulse and 

 hydrospires are directly connected by the ambulacral pores. 



The developement of the recent Crinoid, Antedon rosaceus, as 

 described by Prof. Wyville Thomson (Phil, Trans., 1866), pur- 

 sues a course that could not possibly result in the production of 

 such an animal as Actinocrinus. The pseudembryo, as it is called 

 by Prof. Thomson, is a small ovate organism, with four transverse, 

 ciliated bands, a large key-hole-shaped mouth (pseudostome), and 

 a small circular vent (pseudoproct). These orifices are connected 

 by a rudimentary 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 originates within the pseudembrjo, but 

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

 distinct from those of its nurse. The 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 ot the ambulacral system, shows it to represent the 

 mouth of the adult Star-fish, while that of Actinocrinus 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 paleocrinoids, 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 Actinomnus 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 or 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 paleozoic forms. 



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



