AGAMOGENESIS AMONG POLYCHJ-:'! 215 



thorax of the bud (Fig. 54, /!). S;irs has described a similar 

 mode of multiplication in his pflograna /////*/< .tv/, a \. 



}y allied I'onn. 



In Xt/llitt and in Protula, the producing and the produced 

 zoOids alike develop generative products, but, in Ant,,/ 

 Krohn has shown that the primary produci; 

 sexless, the secondary produced xoi'iids having a M)ine\ 

 different form, and alone giving rise to ova ana /,oa. 



In some species of the genus Nerd*, the v r tin- 



development of its genital organs has taken place, takes on 

 the characters of what was formerly considered a 

 genus, Heteronereis ; and the males and the females of i In- 

 same .species of Nereis have even been reg;> litl.T, m 

 species of Jfeteronereis. 1 



The series of forms represented by the Turbellaria, the 

 Hirudinea,) the Oligochceta, and the Polychatu. illustrates 

 the manner in which a type of organization, which, in its 

 simplest condition, exhibits but little advance upon a mere 

 Gastrula, passes into one in which the body is divided into 

 many segments, each provided with a pair of appendages or 

 rudimentary limbs. 



The segmentation, or serial repetition of homologous 

 somites, extends to the nervous system, and, more or less, to 

 the vascular and reproductive organs, in the higher forms of 

 these "Annuloae" animals; from which a furtber extension 

 of the same process of segmentation, with a fuller develop- 

 ment of the appendages and a more con:pl -e appropriation 

 of some of them to manducatory purposes, leads us to the 

 Arthropoda. 



THE GEPHYREA. These are marine vermil'onn animals 

 without distinct external segmentation or parapodial apprnd- 

 ages. The ectoderm has a chitinmis cuticle, and is often 

 provided with tubercles, hooks, or seta?, of chitin (Echiurus, 

 Sternaspis). No calcareous skeleton is found in any of the 

 hyrea. The integument frequently contains numerous 

 simple glands, the apertures of which perforate the cuticle. 

 In one genus (Sternaspis), two shield-shaped plates, fringed 

 with setae, are developed upon the hinder part of the ventral 

 surface of the body. There are external circular and internal 

 longitudinal muscular fibres beneath the ectoderm. An inner 



Ehlers, " Die Gattung Hetoronertu." ( u GOttingen Nachricliten," 1867.) 



