SPICULES OF SYNAPTA AND AURICULARTA LARVA. 491 



in a litnited nmnber of ceutres — i. e. involves the formation 

 of syncytia, whereas in the Cncumariidee, e.g. in wliicli the 

 spicules are relatively numerous but small, the scleroblasts 

 either remain iaclependent or only associate in pairs. In 

 other words, given a certain number of scleroblasts in the 

 organism, these can either aggregate in some degree to 

 produce a relatively few huge spicules, or can remain inde- 

 pendent and produce numerous small spicules. What factor 

 determines the choice of these alternatives in any given 

 instance I do not pretend to know, and I can only remark, on 

 Spencerian grounds, that the Synapta condition is the more 

 highly evolved. 



With regard to the conspicuously definite disposition of 

 the spicules in the body-wall, the explanation of this feature 

 is, I believe, not difficult to find. In the first place it must 

 be remembered that the body-wall of Synapta is highly con- 

 tractile, the creature being able by meaus of its powerful 

 longitudinal muscles to contract itself with ease to a con- 

 siderable fraction of its normal length. This longitudinal 

 contraction must and does involve a transverse wrinkling of 

 the body-wall, and this contraction not being an infrequent 

 expression of the animal's activity, it seems clear that the 

 initial granule of the spicule will elongate in the transverse 

 grooves formed in the body-wall during such contractions. 

 It is only necessary to mount and examine a contracted 

 portion of the body-wall of Synapta to see that rigid structures 

 like the spicules must lie in the grooves so formed, and no 

 objection can be taken to this view on the ground that the out- 

 growth of the initial granule is appropriately orientated from 

 the very first, since the grooves may be assumed to be quite 

 capable of determining the direction of the initial as well 

 as of the later growth of the spicule. Whether the granule 

 elongates on one side or the other I believe to be purely a 

 matter of chance, the spicules being, as before-mentioned, 

 pretty equally disposed in both directions, as they should be, 

 in the absence of any determining factor. Why the granule 

 only elongates on one side and not on both I am unable to 



