370 ZOOLOGY 



being different from the rest. There thus arises a bilateral sym- 

 metry, there being one vertical plane, and only one that passing 

 through the middle of the madreporite and through the middle of 

 the opposite arm along which it is possible to divide the Starfish 

 into two equal right and left portions. 1 The two rays between 

 which the madreporite lies are termed the bivium, the three 

 remaining the trivium. 



Attached to the spines of the oral surface, in the intervals between 

 them, and in the intervals between the spines of the dorsal surface, 

 are a number of very small, almost microscopic bodies, which are 

 termed the pedicellarics (Fig. 310, Ped). Each of these is supported 

 on a longer or shorter flexible stalk, and consists of three calcareous 

 pieces a basilar piece at the extremity of the stalk, and two jaws, 

 which are movably articulated with the basilar piece, and are 

 capable of being moved by certain sets of muscular fibres so as to 

 open and close on one another like the jaws of a bird. In some of 

 the pedicellarise the jaws, when closed, meet throughout their 

 entire length, while in the case of others, mostly arranged in circles 

 round the spines on the aboral surface, one jaw crosses the other 

 at the end like the mandibles of a Crossbill. Such pedicellarise, 

 with three calcareous pieces, are termed forcipulate. 



In a well-preserved specimen there will be seen in each of the 

 ambulacral grooves two double rows of soft tubular bodies ending 

 in sucker-like extremities ; these are the tube-feet or podia (Fig. 307). 

 In a living specimen they are found to act as the locomotive organs 

 of the animal. They are capable of being greatly extended, and 

 when the Starfish is moving along, it will be observed to do so by 

 the tube-feet being extended outwards and forwards (i.e. in the 

 direction in which the animal is moving), their extremities becom- 

 ing fixed by the suckers, and then the whole tube-foot contracting 

 so as to draw the body forwards ; the hold of the sucker 

 then becomes relaxed, the tube-foot is stretched forwards again, 

 and so on. The action of all the tube-feet, extending and con- 

 tracting in this way, results in the steady progress of the Starfish 

 over the surface. With the aid of the tube-feet the Starfish is 

 also able to right itself if it is turned over on its back. 



At the extremity of each of the ambulacral grooves is to be 

 distinguished a small bright red speck, the eye (Fig. 310, A, oc), 

 over which is a median process, the tentacle (t), similar to the tube- 

 feet, but smaller and without the terminal sucker. The tentacles 

 have been ascertained by experiment to be olfactory organs, the 

 Starfish being guided to its food much more by this means than 

 by the sense of sight. 



Transverse Section of an Arm. If one of the arms be cut 

 across transversely (Fig. 308 and Fig. 310, B) and the cut surface 



1 The slightly ecrenlric position of ihe anal aperture introduces a corre- 

 spondingly slight inequality between the right and left portions. 



