AGAMOGENESIS AMONG POLYCILETA. 215 



thorax of the bud (Fig. 54, JB). Sars has described a similar 

 mode of multiplication in his Jbilograna implexa, a very close- 

 ly allied form. 



In /Syllis and in Protula, the producing and the produced 

 zooids alike develop generative products, but, in Autolytus, 

 Krohn has shown that the primary producing zooid remains 

 sexless, the secondary produced zooids having a somewhat 

 different form, and alone giving rise to ova and spermatozoa. 



In some species of the genus Nereis, the worm, after the 

 development of its genital organs has taken place, takes on 

 the characters of what was formerly considered a distinct 

 genus, Heteronereis ; and the males and the females of the 

 same species of Nereis have even been regarded as different 

 species of Heteronereis. 1 



The series of forms represented by the Turbellaria, the 

 Hirudinea, the Oligochceta, and the Polychmta, 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 

 mam' 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 " Annulose " animals ; from which a further extension 

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

 ment of the appendages and a more complete appropriation 

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

 Arthropoda. 



The Gephyeea. — These are marine vermiform animals 

 without distinct external segmentation or parapodial append- 

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

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

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

 Gephyrea. 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 



1 Ehlers, " Die Gattung Heteronereis.^ ( u Gottingen Nachrichten," 1867.) 



