VON BUCII ON THE CYSTIDEA. 29 



of one main direction or tendency, combined and brought into action 

 by two directions or tendencies opposed to another. In one species 

 of Pentremite from Yorkshire, which is not yet named, but which 

 has very flattened ambulacra, a species, it will be observed, of a 

 genus whose very name is characteristic of the division into five parts, 

 we may clearly observe one principal arm more especially elevating 

 the plates, and two other arms at the side, each dividing itself to 

 form the posterior pair which complete the five rows of ambulacra. 

 Is not this the case also in Spatangus^ Clypeaster, and even the re- 

 gularly circular Cidaris? and may not possibly the study of man 

 and other Vertebrata — the head being the main development, the 

 arms the opposed tendencies, and the legs the posterior pair derived 

 from the anterior but detached by the prolongation of the vertebral 

 column, — may not, I say, this lead to the same conclusions as those 

 derived by the investigations of the Invertebrata, which I believe 

 have been already frequently put forth ? 



I turn now to the nearer consideration of the Cystidea. 



The Cystidea were natural bodies supported on a stem or pedicle 

 which was attached to the ground; their surface, more or less sphe- 

 rical, was covered by a great number of polyhedral plates accurately 

 fitted to one another, and between these plates were certain open- 

 ings necessary for the performance of the animal functions ; but from 

 none of these did arms proceed resembling those of the Crinoidea. 

 The animal was completely without arms. 



With regard to the openings on the surface, we find in all the 

 Cystidea, — 1st, that the mouth was planted in the central part of the 

 upper surface, generally in a moveable proboscis covered with mi- 

 nute plates ; 2nd, that besides this mouth, and close to it, there is 

 generally, if not always, a small anal orifice penetrating the plate, 

 but not itself surrounded with any plates peculiar to it ; 3rd, that 

 further towards the middle, but almost invariably on the upper half 

 of the body on which the mouth is placed, there rises a round or oval 

 aperture, not connected with the mouth, and often covered by a 

 five or six-sided pyramid, which seems to be composed of as many 

 little valves. This probably forms the ovarial orifice of the animal. 



These openings, with the exception of the mouth, are not found 

 to exist when arms begin to be developed from the upper surface ; 

 and we may easily understand this when we remember that in the 

 latter case the ovaries are carried out with the arms beyond the rim 

 of the cup-like body, so that a separate opening for them would be 

 useless. In all the Cystidea the presence of these ovarial orifices is 

 however manifestly essential. 



Since scarcely anything appears of the interior of these animals, 

 and one can only observe their external structure, it is not remark- 

 able that the number six appears especially to predominate, while 

 the quinary arrangement is rarely observed. The latter may, how- 

 ever, be traced in the stem, and in the internal nutritive canal run- 

 ning through the stem, which I have never seen other than fivc- 

 sided. The stem itself is in almost all the genera remarkably 



