SOME PROBLEMS OF ANNELID MORPHOLOGY. 69 



PoIygQvdiiis, which gives rise to muscles, blood-vessels, 

 etc., and which, by segmentation, first blocks out the 

 metamerism of the trunk. Thus, with exception of the 

 alimentary canal, every system of the body — circula- 

 tory, excretory, muscular, nervous, reproductive — is 

 laid down in two completely separate halves. And the 

 union of the two germ-bands, which form the two halves 

 of the trunk, is a typical and unquestionable case of 

 concrescence. This extraordinary phenomenon is ex- 

 hibited in its greatest perfection in the leeches and 

 some of the fresh-water annelids (" naids "). It occurs 

 in a striking form in the development of the earth- 

 worm, though modified by the very different structure 

 of the gastrula. In Polygordiiis the two halves of the 

 body are never as completely separated as in the leech, 

 yet the primary separation and subsequent growing 

 together of the mesoblastic bands is clearly enough a 

 simplified form of the same general phenomenon ; and 

 the same is true of many other marine annelids. Among 

 the arthropods complete concrescence, — i.e. the com- 

 plete separation of the two halves of the body on the 

 ventral side — has been observed in a single case only ; 

 but a partial concrescence, comparable with that of 

 Polyg07'dius, probably occurs throughout the entire 

 group. Whether complete concrescence occurs among 

 the vertebrates or not is still a disputed question. It 

 is asserted, on very high authority, to take place in 

 some of the lowest vertebrates (sharks and bony fishes) 

 in nearly as typical a form as among the leeches, but 

 this is disputed by many observers. It is, however, 

 unquestionable that a partial concrescence — that for 

 instance in the mesoblast and the central nervous sys- 



