Polyzo 



179 



end of tlie ctitinous case, a movable, chitinised fold of the wall, which 

 acts as an operculum to the mouth of the tentacle-sheath, when 

 the soft part of the hody is retracted. The m o u t h is at the anterior 

 end in the midst of the circle of tentacles ; the anus lies at this end 



Fig. 140. A — B Diagrammatic longitudinal sections of a Polyzoon, A expanded, 

 B retracted, a auns, b hind-end, e rectum, / fore-end, I operculum, m stomaoli, n nerve 

 ganglion, o mouth, s oesophagus, t tentacle. The chitiuous covering is indicated by a wide 

 black line, the soft wall of the body is shaded, C avicularia (diagrammatic), I oper- 

 culum, m its muscles, ta gut. — Orig. 



also, not far from the mouth, and is usually just without, seldom 

 within, the lophophore. The alimentary canal is, therefore, in 

 the form of a loop ; it is made up of an oesophagus, a stomach 

 provided with a caecum, and a rectum. The food, consisting of 

 microscopic particles, is driven into the mouth by the cilia of the 

 tentacles. The central nervous system consists of a nerve 

 ganglion, which is situate on the side of the oesophagus near the anus, 

 and of a nerve-ring surrounding the oesophagus. Nerves from the 

 ganglion pass to the different parts of the body. Optic and auditory 

 organs are wanting ; so are a vascular system, and special respiratory 

 organs ; the lophophore is, however, doubtless of respiratory import- 

 ance. Excretory organs have hitherto been found in only a few 

 Polyzoa, in the form of two short canals, opening at one end into the 



N 2 



