36 



LECTURE III. 



jaw. The efficacy of these instruments in tearing to fragments the 

 objects swallowed may be easily discerned in the living animal 

 through its transparent parietes. 



The condition of the alimentary canal is very similar in most of the 

 genera, which are chiefly distinguished thereby from the Polygastric 

 Infusoria. It is a more or less simple tube (Jig. 15, //), extending longi- 

 tudinally through the well-developed abdominal cavity, to terminate by 

 a cloacal outlet (A) at the hinder end of the body, generally above the 

 base of the sheath of the claspers. It is sometimes wider, sometimes 

 narrower, sometimes with and sometimes without a constriction indi- 

 16. cative of the stomach {Jig. 15, g) : in Rotifer- (Jig.16.} 



and Ptyura there is a distinct terminal dilation or rec- 

 tum ; sometimes the intestine is complicated with many 

 caeca, as in Diglena and Megalotrocha. Most of the 

 species have, just behind the pharynx, or continued 

 from the stomach, two large oval glandular sacs, rarely 

 cylindrical or bifurcated, to which sometimes fila- 

 mentary casca are appended, as in Enteroplea. These 

 secerning sacs {Jig. 15, i) may discharge the office of 

 liver or salivary glands. 



Ehrenberg recognises a vascular system in the pa- 

 rallel transverse slender bands which surround the 

 body ; these are in close connection with the integu- 

 ment. With more probability we may regard as san- 

 guiferous organs the free longitudinal vessels, likewise 

 indicated by Ehrenberg, on the dorsal as^ject, which 

 are connected with a fine vascular network near the 

 mouth, and which send filamentary tubes to the intestine. 



The wheel-like organs, by rapidly changing the oxygenated fluid 

 which bathes their surface, may be supposed to take the most potent 

 share in the respiratory function. But Ehrenberg directs our attention 

 to some peculiar ciliated vibrating oval corpuscules, which are attached 

 to the free seminal tubes on each side the abdomen ; and to which cor- 

 puscules he assigns the name of internal gills. The water essential to 

 the respiratory function discharged by these problematical bodies, and 

 by the vascular surfaces of the viscera, is admitted into the interior of 

 the body by an opening in the neck, which, in very many species, is 

 prolonged upon one or two spear-shaped tubes, which are beset with 

 vibratile cilia : the water is observed to pass to and fro in streams 

 through these tubes. It consists of an expanded part and an ap- 

 pendage ; the expanded part consists of three folds or vesicles ; the 

 vibrating appendage resembles a crotchet in music. 



The Rotifera are androgynous : most of the species are oviparous, 



