392 THE ANATOMY OF INVERTERBATED ANIMALS. 



The single nervous ganglion is situated, as has been 

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

 laria^ Scrupocellaria, and some other genera, nervous cords 

 and plexuses connecting the ganglia of the several polypides, 

 and constituting what F. Mliller ^ terms a " colonial nervous 

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

 these cords and plexuses are really nerves. It is doubtful if 

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

 — the ejnstoma — 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- 

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

 ments, which, when 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. Ili. — ScriipocePMria fernx.—\ small portion of a polyzoarlum, Bhowing tlie 



vibracula (a). (After Busk.) * 



J " Arclilv liir Anatomie," 1860. 



2 Farre, " Ohservations on the Minute Stnictiire of some of the Hisrher Forms 

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

 (" Ahh. d. konisrl. Akad. der Wissenschaften," Berlin, 1869). 



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

 Btomata," 1852-'o4. See for this prroup Nitsche's recent important "Beitrage zur 

 Kenntniss der Bryozoen" {Zeitachrift f>'ir wiss. Zoologie^ 1869-'71). 



* " Catalogue of Marine Polyzoa," 1852. 



