Chap. II. HOMOLOGICAL RELATIONS. 69 



The limits of their extension mark the bomiclaries of that area of the spherosome, 

 which I have called the actinal area, and the complication of their ramifications 

 characterizes the different zones of this area, and the various fields of each zone. 

 In Synapta, for instance, there arise a number of digitate appendages from the 

 ring encircling the mouth, which are quite characteristic of that family, while the 

 radiating tubes, upon the sides of the tubular hody, are simple, and destitute of 

 ambulacral suckers. In Pentacta and Cuvieria, the appendages around the mouth 

 assume the character of complicated and highly ramified tentacles, while the radi- 

 ating tubes are provided with ambulacral suckers, varying even in different rows. 

 In Echiuoids, the differences in the structure of the ambulacra are much greater, 

 in different ftimilies, than among the Holothurians : in Echinus and Cidaris, the 

 five zones have identical ambulacra, though in each zone the ambulacral suckers, 

 and the other appendages of that system, differ with their distance from the cen- 

 tre of radiation; in Echinolampas, and still more in different genera of Spatangoids, 

 the zones of ambulacra differ among themselves, and each zone within itself; but 

 in all they extend, as in the Holothurians, nearly over the whole surface of the 

 body, with the exception of a small abactiual area opposite the mouth. 



Not so in the Starfishes. Here the ambulacra occupy only a narrow space 

 on the lower surface of the body, while the abactinal area occupies the whole upper 

 surface and the sides of the arms. The ambulacral or actinal area is, indeed, very 

 similar in all the Asteroidse. It is uniformly composed of a In-oad, double series 

 of ambulacral plates, between which project the ambulacral suckers, and a narrow 

 series of interambulacral plates on each side of the former, both kinds of which are 

 larger about the mouth, and gradually smaller towards the extremity of the rays. 

 The abactinal area varies much more; and while in some it is occupied by very 

 similar plates, forming a more or less open net-work, in others it presents the 

 most diversified combinations of heterogeneous plates, regularly linked together in 

 distinct rows or well defined and distinct fields. And yet nothing is easier than 

 to transform an Asterias into an Echinus. It is only necessary to contract the abac- 

 tinal area of any Starfish, to such an extent, that the ambulacral area may be 

 curved upwards, and the interambulacral plates, on opposite sides of adjoining fur- 

 rows, meet ; or to stretch the abactinal area of a Sea-urchin to such an extent, that 

 the extremity of the ambulacra, with the ocellar plate, are brought to a level with 

 the plane of the mouth. In this position, the abactinal area of an Echinus may 

 directly be compared to that of an Asterias, and the latter with a Discophorous 

 Acaleph. Whether the circular tube, connecting the ramifications of the chymiferous 

 tubes, be at the peripheric extremity of the system, as in Aurelia, or around the 

 mouth, as in Idyia, or half way between the mouth and the abactinal area, as in 

 the ScutellidjB, does not alter their homologies. 



