Characteristics of Polyzoa. ciii 



of the retractor and protractor muscles of the entire polypide. The 

 nerve mass situated between rectum and oesophag-us^ has been 

 fig-ured and described as sending filaments to the lophophore^ ten- 

 tacles, epistome, digestive tract, evaginable endocyst, and retractor 

 muscles, and as also throwing a collar round the oesophagus ; and 

 it may consequently be considered as representing both the parieto- 

 splanehnic and the cerebroid ganglia of higher Molluscs. Repro- 

 duction in the Polyzoa is both sexual and asexual. The asexual 

 takes place in the way of gemmation ; and in the case of the 

 Fresh- water Polyzoa by means of ' statoblasts^ or gemmae, in which 

 the developmental activity remains latent for a period. The Polyzoa 

 are hermaphrodite ; the testes being situated near the bottom, the 

 ovary being attached to the parietes of the upper part of the cell. 

 It is by gemmation that the polymorphic organisms known in 

 Polyzoa as ' ovicells,^ ' avicularia/ and ' vibracula^ are produced ; 

 and in Serialaria, where the entire colony may have its individual 

 polypide brought into connection by a common nerve-system, we 

 find some cells modified for the discharge of purely passive functions, 

 as ' stem-'' cells and ' root-' cells. 



The Polyzoa have, like the Tunicata, been removed from the Sub- 

 kingdom Mollusca by Gegenbaur and Haeckel, and classed with the 

 Vermes. The inosculant foi-m Loxosoma described by Kowalewsky 

 (Mem. Acad. Imp. St. Petersburgh, Ser. vii., Tom. x. 2) has been sup- 

 posed to connect them with this latter Sub-kingdom. Against this view 

 we must set not merely the many sti'uctural homologies which can be 

 pointed out as existing between adult individuals of the undoubtedly 

 MoUuscoid Classes Brachiopoda and the Polyzoa ; (for which see p. 72, 

 infra, ibique citato), but also the striking shnilarity which the larval 

 form of a Brachiopod has been observed by Fritz Miiller to present to 

 the Polyzoa with ox-bicular lophophores such as all the marine sub- 

 orders Avith the exception of Pedicellinea. See Reichert und Du-Bois 

 Reymond's Archiv. fiir Anatomic und Physiologic, p. 79, i860. 



For an account of the nervous system in Serialaria, see Fritz Miiller, 

 Archiv. fiir Naturgeschichte i860, p. 311, Taf. xiii. 



