30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



somites anterior to the last one, though each of these somites has a 

 pair of simple protonephridia. In the terminal somite are formed 

 the reproductive organs, which, in the female, consist of a delicate 

 gonadial sac, either single or double, extending forward in the body, 

 and opening posteriorly by a median pore, at least at the time of egg 

 laying. The gonadial sac appears to represent the coelom of the last 

 segment, though, as Iwanofif (1928) points out, it may be simply a 

 space accommodating the germ cells in the undifferentiated tissue 

 near the end of the body. Hence, the apparent last somite is either 

 a single teloblastic genital somite, or a region corresponding with 

 the zone of growth of the polymerous annelids. 



A concrete example of the secondary distribution of the germ cells 

 in a polymerous annelid is given by Malaquin (1924 a) in his study 

 of the development of SaUnacina dysferi. The germ cells, as we have 

 seen, are first localized in the growing zone of the young larva. When 

 the formation of the postlarval segments begins, Malaquin says, the 

 germ cells multiply, and three, four, or five of the resulting gono- 

 cytes become adherent to the outer wall of each new coelomic sac. 

 Thus the germ cells, proliferated from a constant source, are dis- 

 tributed to the newly forming somites, and are extracoelomic both in 

 their origin and in their secondary segmental positions. After a 

 period of inactivity the segmentally distributed gonocytes begin to 

 multiply in the coelomic walls, and here form the small masses of 

 germinal cells ensheathed in peritoneal folds that are known as the 

 "gonads." 



If, now, the ontogenetic facts of annelid growth are given a phylo- 

 genetic significance, it becomes evident, as claimed by Iwanofif (1928), 

 that the extension of the worm by the teloblastic generation of new 

 somites, in which are apportioned groups of the multiplying germ 

 cells, was primarily a means of amplifying the reproductive function. 

 In the course of evolution it gave rise to a type of animal from which 

 have been derived the modern Annelida, the Onychophora, and the 

 Arthropoda. 



The teloblastic genital segments are in many respects mechanical 

 improvements over the primary segments; their muscular equipment 

 is stronger, the parapodia better constructed for locomotion, the 

 dissepiments usually more complete, and the nephridia more efficient 

 for excretory purposes. Hence, the whole worm is clearly a stronger 

 and a more active animal by reason of the addition of the well- 

 organized reproductive somites. At the bottom of the water the 

 creeping worm is better able to force its way under stones or into 

 crevices, or to burrow into sand or mud; but at the breeding season 



