348 PROCEEDINGS OP THE ACADEMY OF [1887. 



The presence of two lateral rows of vestigiary structures, iu the 

 position of parapodia, on either side of the tail of embryos of certain 

 JElasmobranchs, and on either side of the dorsal median line, oppo- 

 site the interval where the vertical dorsal fins are wanting, as first 

 described by P. Mayer, led that investigator to regard such vestiges 

 as parapodoid in character. 



The existence of vestigiary structures of this kind, as well as 

 their segmented or metameric relations, which are essentially the same 

 in their general features, as those of the metameric elements of the 

 limbs themselves, lends additional probability to the doctrine, that 

 the vertebrates are the off-shoots of a worm-like ancestral form. 



The suppression of the caudal parapodia or radial elements of the 

 fins in the ancestral types, which led to the differentiation of the 

 paired fins, may be ascribed to a number of other influences be- 

 sides those referred to by Dohrn, of which the following may be 

 mentioned, 



1. If the ancestral type has been one which was at one time 

 tubicolous in habit, the abortion of the caudal parapodia would be 

 accounted for. Though, it must be admitted that this view is in 

 the highest degree improbable, but not impossible. 



2. The late outgrowth of the tail in annelids as well as in verte- 

 brates, presupposes a belating of the functional activity of the cau- 

 dal parapodia, through which their importance and development 

 may have been hindered. 



3. The slight use made of the parapodia of the tail as locomo- 

 tive organs, after the tail became laterally flattened or compressed, 

 since it is well known that the principal power of propulsion exerted 

 by fishes, is by means of the alternate flexures of the tail ; the fins 

 aiding only in a moderate degree, their principal office being to bal- 

 ance or hold the body and keep it in the normal position. The ex- 

 ceptions to this rule are very few ; the most important being that of 

 Mola in which the dorsal and anal fins are the sole organs of loco- 

 motion, as shown by the writer elsewhere. 



The abortion of the caudal part of the intestine, now represented 

 by the post-anal gut, as it is termed by embryologists, may have 

 had something to do with the development of paired limbs over the 

 sides of what remained of the body cavity, as urged by Dohrn, but 

 there must have been forces at work which led to thia abortive pro- 

 cess, by which the primitive gut was shortened. And, while it is 

 undoubtedly a fact that such a shortening of the alimentary tract 



