288 Mr. T. Austin on the Connexion between 



genital openings however differ in position. Those of the Pen- 

 tremites encircle the mouth, whilst those of the former are margi- 

 nal ; yet here we have links which connect the Pentremites with 

 listerias and Ophiura. 



By investigating the family of Pentremites we are led pro- 

 gressively, as before indicated, to the Echinidce, when, taking Ci- 

 daris for the type, we find the skeleton formed of five tumid or 

 blunt rays, each composed of a double series of hexagonal plates, 

 these rays being separated by a similar number of ambulacra, 

 sinuous and perforated by minute foramina or ambulacral pores. 

 The ambulacra radiating from the mouth, which is beneath, and 

 taking an upward course, terminate near the apex, which is com- 

 posed of five plates, each of which has a central opening or ovarial 

 aperture. These pieces united may be considered as the dorso- 

 central plate, in the centre of which the vent is situated. 



From this genus we pass by natural gradations to others ; but 

 we may pause for an instant to remark on the resemblance which 

 exists between the genus Conulus and some species of Pentremites, 

 in which the ovarian openings exhibit considerable similitude to 

 each other. 



As previously observed, in Cidaris and Echinus the mouth is 

 found beneath, and the anal opening above or opposite ; but in 

 Spatangus and other allied forms the anal pore occupies a lateral 

 position, and as we extend our observations we find it, by gradual 

 modifications in different genera or species, becoming more 

 remote from the dorsal plane, and consequently by degrees ap- 

 proaching the oral aperture. Thus in Clypeaster its position is 

 in the extreme margin, in Echinoneus it is seen on the same plane 

 as the mouth, and finally its approximation becomes so close, that 

 at length in some instances it is merged in the oral opening as 

 in Echinocyamus, thus returning through various and beautiful 

 modifications of form and structure to the starting-point, or until 

 it becomes typical with the Star-fishes. 



If a common Star-fish of five rays or lobes be examined, we 

 find it to have a well-defined ambulacrum on the inner surface, 

 sunk as it were in a deep furrow, which diverges from the central 

 disc in the direction of the rays. Numerous foramina and small 

 spines will also be observed. By flattening out the rays, the 

 perforated ambulacra will be found ranged along the sides ; and 

 if we bring up the points of the Star-fish to meet in a centre 

 above, a spherical body is at once produced closely resembling an 

 Echinus, the now curved ambulacra meeting and forming the 

 double lines as seen most clearly in Cidaris ; and further, if the 

 extreme points of the star are turned inwards, an apophysis is 

 formed capable of supporting the muscles which in Echinus move 

 and sustain the maxillary process or jaws. 



