32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



loaded with the mature generative elements. The reproductive tail- 

 ends (epitokes) then actively swim to the surface, where myriads 

 of them congregate to liberate the gametes. In their accustomed 

 haunts the anterior nonreproductive sections (atokes) regenerate the 

 discarded epitokes in preparation for next year's consignment to the 

 breeding grounds. Various other species of Polychaeta have similar 

 habits. The Syllidae are famous for the many forms of schizo- 

 genesis, strobilation, and budding that take place among them, but 

 here the detached piece, either before or after separation, generates 

 a new head and becomes a complete worm except perhaps for the 

 lack of an alimentary canal and a few other unimportant structures. 

 Again, in some of the Ctenodrilidae the worm breaks up by con- 

 striction into several pieces of a few segments each, and the middle 

 pieces regenerate both a head and a tail. 



The periodic fragmentation of the body for reproductive purposes, 

 however, cannot lead to anything in the way of constructive evolution, 

 and, with the annelids in general, the tendency has been to integrate 

 the entire series of somites into a mechanical and physiological unit, 

 in which the reproductive cells are assigned to definite segments. In 

 the Arthropoda, though the body may still be composed of freely 

 movable segments, the process of integration has been carried so far, 

 and the various organs so interdependently distributed, that fission 

 becomes impossible without fatal results. It would seem, therefore, 

 that the teloblastic somites, first added apparently for reproductive 

 efficiency, have been found so useful in other ways that they have 

 come to constitute not only the largest part of the body in all the 

 articulate animals, but its most important part, except for the primary 

 sensory and nervous elements contained in the head. 



A structural differentiation between groups of somites, forming 

 distinct body regions, or tagmata, has taken place in many of the 

 polychaetes, particularly in the Sedentaria, and is a characteristic 

 feature of all the Arthropoda. The zone of growth, therefore, which 

 presumably at first gave rise to a series of identical somites, has 

 acquired the remarkable faculty of differential activity, producing 

 successively, at definite segment intervals, two or more series of 

 somites having often a strongly contrasting structure, while minor 

 differences may be distributed throughout the entire series of 

 segments. 



THE PROSTOMIUM AND ITS APPENDAGES 



The annelid prostomium is the part of the trunk that is not invaded 

 by the blastopore as the latter elongates forward on the ventral sur- 



