XIV INTRODUCTION. 



and Nitsche * ; they lie without the limits of the present 

 work. 



The structure of the polypide as it occurs in the ordi- 

 nary marine forms may be considered under the following 

 headings : i. the tentacular crown and sheath ; ii. the 

 alimentary canal ; iii. the nervous system ; iv. the mus- 

 cular system. 



TENTACULAR CROWN AND SHEATH. The tentacles form 

 a bell-shaped wreath, which is borne on a kind of circular 

 stage (the lophophore), occupying the summit of the body, 

 and perforated in the centre by the mouth. They are 

 ranged round the circumference of the lophophore, which 

 constitutes at once the floor of the tentacular bell and the 

 roof of the perigastric cavity, lying immediately below it. 

 The mouth is a simple round orifice, opening into the 

 oesophagus, and so placed as to form the focus towards 

 which the supplies of food drawn in by the ciliary vortex 

 converge. The slender filiform tentacles are tubular, 

 closed at the free extremity, and opening at the base into 

 the perigastric cavity f, or space interposed between the 

 wall of the cell and the body of the polypide, from which 

 the nutritive fluid is freely admitted to them. They are 

 furnished with vibratile cilia, which are ranged in a 

 single line along the two opposite sides, and by their rapid 

 and incessant movement, when the polypide is expanded, 

 create a very maelstrom in the water, which sweeps the 



* "Beitrage z. Anat. u. Entwicklungsgesch. d. phylactolocm. Siissw. 

 Bryoz.," Archiv f. Anat. u. Physiol. 1868. Sep. Abdr. " Ueb. die Knospung 

 der Polypide der phylactolaemen Susswasserbryozoen," Zeitech. f. wisseusch. 

 Zool. xxv. Suppl. Bd. Heft 3, 1876. 



t There seems to be little doubt that this is the case, though Reiohert 

 describes the cavity of the tentacles in Zoobotryon as being in communica- 

 tion by mran.s of minute orifices with that of the oesophagus. 



