392 THE ANATOMY OF INVERTERBATED ANIMALS. 



The single nervous ganglion is situated, as has been 

 stated, between the oral and the anal apertures. In iSeria- 

 laria, Scrupocellaria, and some other genera, nervous cords 

 and plexuses connecting the ganglia of the several polypides, 

 and constituting what F. Muiler' terms a "colonial nervous 

 system," have been described. But it is not yet certain that 

 these cords and plexuses are really nerves. ]t is doubtful if 

 there are any special organs of sense, unless a lobed process 

 — the epistoma — which overhangs the mouth in many fresh- 

 water Polyzoa, be of this nature. The ectoderm of that region 

 of the body which lies immediately beneath the tentacula is 

 always soft and flexible ; and when the lophophore is re- 

 tracted, becomes invaginated, so as to form a sheath, by which 

 the tentacles are protected. Sometimes, as in the Ctenosto- 

 mata^^ this sheath is surrounded by a circle of chitinous fila- 

 ments, which, w^hen the tentacles are retracted, furnish a pro- 

 tective outer covering to them. And, sometimes, as in the 

 Cheilostomata,^ part of the ectocyst of the polype cell is dis- 

 posed in such a manner as to constitute a movable lid, which 



Fig. \\i. — Scrvpocellaria ferox.—k smal] portion of a polyzoariura, showing the 

 vibracula (a). (After Busk.) * 



1 " ArcViiv liir Anatomie," 1860. 



2 Farre, " Ohservations on the Minute Rtruotnre of some of the Hisfher Forms 

 of Polypi" ( " Phil. Trans.," 1837\ Reichert, " Ueber Zonbofryon pellucidus''' 

 (" Ahh. d. konid. Akad. der Wissenschaften," Berlin, 18R9). 



3 Busk, " CatalofTue of the Marine Polyzoa in the British Museum: Cheilo- 

 Stomata," 1852-'54. ''See for this ^roiip Nitsche's recent important ''Beitrage zur 

 Kenntniss der Bryozoen" {ZeiUchriftfi'ir wiss. Zoologie, 1869-'71). 



4 " Catalogue of Marine Polyzoa," 1852. 



