SCOLECIDA. 



125 



partitions. The integument is usually provided with bundles of 

 muscular fibres taking a longitudinal and transverse direction. In 

 the free forms the anterior ciliated disc acts somewhat like the pro- 

 peller of a screw-steamer in driving the organism through the water 

 — in all cases it has the action of producing currents in the water by 

 which particles of food are brought to the mouth. The posterior 

 end of the body is usually developed in the free forms into a kind 

 of tail or foot (fig. 79, A), which may take the shape of a kind of 

 pincers or of a little suctorial disc. 



As regards their internal anatomy, in the females of almost all 



Fig, 79. — Rotifera. A, Diagrammatic representation of Hydatina senta (generalised 

 from Pritchard) ; a Depression in the ciliated disc leading to the digestive canal : 

 b Mouth ; c Pharyngeal bulb with masticatory apparatus ; d Stomach ; e Cloaca ; / 

 Contractile bladder ; g g Respiratory or water-vascular tubes ; h Nerve-ganglion, 

 giving filament to ciliated pit (k) ; o Ovary. B, Melicerta ringetis (after Gosse). 



the Rotifera there is a well-developed alimentary canal, which is 

 completely shut oif from the general cavity of the body. The 

 inouth (fig. 79,. A, b) opens into » dilated chamber or " pharyngeal 

 bulb" (c), which contains a complicated apparatus of horny teeth. 

 The pharynx opens into a capacious stomach {d), continued into an 

 intestine which terminates by a chamber known as the " cloaca " (e), 

 which forms the common outlet for the water-vascular and genera- 

 tive systems. In both sexes there is a well-developed water-vascular 

 system consisting of a contractile chamber or bladder (/), opening 

 into the cloaca, and giving origin to two complicated tubes which 



