94 MOUPIIOLOGICAL DEVELOPMENT. 



We have now to study annulose animals in general. Com- 

 parison of them will disclose various phases of progiessivo 

 integration of the kind to be anticipated. 



Among the simpler Annuloida, as in the Nemertidoi and in 

 some kinds of Planaria, transverse fission occurs. A por- 

 tion of a Planaria separated by spontaneous constriction, 

 becomes an independent individual. Sir J. G. Dalyell found 

 that in some cases numerous fragments artificially separated, 

 grew into perfect animals. In these annuloids which thua 

 remind us of the lowest Hydrozoa in their powers of agamo- 

 genetic multiplication, the individuals produced one from 

 another, do not continue connected. As the young ones 

 laterally budded-off by the Hydra separate when complete, 

 80 do the yoimg ones longitudinally budded-ofi' by the Pla- 

 naria. Fig. 166 indicates this. But there are allied types 

 which show us a more or less persistent union of homologous 

 parts, or individuals, similarly arising by longitudinal gem- 

 mation. The cestoid Entozoa furnish illustrations. "Without 

 dwelling on the fact that each segment of a Twnia, like each 

 separate Planaria, is an independent hermaphrodite, or on the 

 fact that both develop their ova by the peculiar method of 

 forming germinal vesicles in one canal and surrounding them 

 with yelk that is secreted in another canal ; and without 

 specifying the sundry conunon structural traits which add 

 probability to the suspicion that there is some kinship be- 

 tween the individuals of the one order and the segments of 

 the other ; it will sufiice to point out that the two types are 

 so far allied as to demand their union under the same sub- 

 class title. And recognizing this kinship, we see significance 

 in the fact that in the one case the longitudinally-produced 

 gemmae separate as complete individuals, and in the other 

 continue united as segments in smaller or larger numbers 

 and for shorter or longer periods. In Tcenia echinococcits, 

 represented in Fig. 167, we have a species in which the 

 number of segments thus united does not exceed four. In 

 Echinohothrium typus there are eight or ten ; and in cestoids 



