392 COMPARATIVE ANATOMY chap. 



spines arranged in a row, ivhicli runs up from the ventral to tlie dorsal side of the 

 arm ; these spines are connected by a thin, transparent membrane in such a way as 

 to form a sort of fin (Fig. 339). On the first three free joints of the arm, the fin of 

 one side of the arm passes into that of the other side dorsally. We are justified 

 in assuming that the animal is able to swim by means of these large fins on 

 the arms. 



The genera OjjMotholia and Ophiohdus are distinguished by peculiar umbrella- 

 shaped spines. A stem with a swollen, button -like base, articulating with a 

 tubercle, carries at its tip a circle of recurved spines, which, during life, are covered 

 by a common integument. These are found either in groups near the base of the 

 ordinary brachial spines, as in OpMotholia, where they first appear at some distance 

 from the disc, or else replacing the ordinary spines near the end of the arm, as in 



Function of the spines. — The fact that the spines serve princi- 

 pally for the protection of the body is at once evident, especially when 

 they are pirovided with poison glands. 



In response to stimuli, the spines become erect. In Diadema setosum, which is 

 very sensitive to light, the long spines turn threateningly towards a hand which 

 approaches them from any side. The spines of most Echinoids further serve for 

 locomotion, moving in a co-ordinated manner. This has been directly proved in 

 the Cidaridcc, Arhacia, Echinus, and Spatangus, etc. In the first of these forms, 

 the long (principal) spines are indeed the chief, or the only, locomotory organs, and 

 are used as stilts. Many Echinoids, e.g. Dorocidaris, Arhacia, Spatangus, if laid 

 on their backs, can turn themselves over again by the help of their spines. 



It has also been proved that the spines may serve for seizing prey and for 

 forwarding it to the mouth. Several spines incline towards the prey, seize it with 

 their tips, and pass it on to the next group in the oral direction, and so on towards 

 the mouth. Compare with this the rise of pedicellarife in Asteroids, p. 394. 



2. Modified Spines. 



a. The Sphseridia of the Eehinoidea. — These are small spherical 

 or ellipsoidal bodies, which, by means of a short stalk, articulate with 

 a prominence of the test, and are inclined sometimes in one direction, 

 sometimes in another. They either project freely, or else rise from the 

 base of a pit-like depression of the test (Fig. 340). This depression 

 may more or less completely close over the sphaeridium. We are 

 here reminded of the various forms of acoustic tentacles in the 

 Medusae, which sometimes rise freely from the body, sometimes from 

 the base of a pit, sometimes on the walls of closed vesicles, which latter 

 come into existence through the concrescence of the edge of the pit 

 above the tentacle. Here, however, we have to do not with tentacles, 

 but evidently with modified spines. 



Sphseridia occur in all Eehinoidea except the Cidaroida. They are found only 

 on the ambulacra, and always on the peristomal plates, although in many forms 

 they are not limited to these, the area in which they occur stretching out in the 

 direction of the ambitus, or even beyond it. The number and arrangement of the 

 sphseridia vary greatly in different groups. 



