IV VERMES— INTESTINAL GANAL 209 



end of the body, and then bends round to run forwards as an ascend- 

 ing limb. The two limbs twist round each other and form a spiral. 

 A ciliated channel runs along the whole course of the mid-gut. Where 

 this ceases, at its posterior end, a blindly closed diverticulum is 

 attached to the gut, winding round its end ; this may be homologous 

 with the accessory intestine of the Ghcetopoda. 



The intestine of Phoronis, like that of the Sipunculacm, forms a 

 descending limb, and on the dorsal side of this an ascending limb. 



The arrangement in the Bryozoa (Fig. 139) is closely connected with 

 that in the already described Prosopygia. A descending limb of the 

 intestine, not very sharply distinguished from the fore-gut, leads into 

 the expanded stomach, from which arises an ascending limb which 

 passes into the hind-gut. The stomach is occasionally prolonged into 

 a somewhat sharply demarcated caecum directed backwards. 



In the mid-gut of the Brachiopoda we distinguish an anterior widened 

 part, the stomach, into which the oesophagus enters, and an adjoining 

 narrower part, the stomach-intestine. The stomach carries one or more 

 pairs of lateral diverticula which branch and divide into massive 

 glandular lobes, called the liver ; these envelop the stomach on all 

 sides. The stomach-intestine forms either a simple or complicated coil 

 and then runs backwards {Crania), or bends on one side round to the 

 front (the other Ecardines), or ends blindly (Testicardines). 



In Dinophilus (Fig. 162, p. 246) and the Rotatoria (Fig. 123, p. 

 185; Fig. 161, p. 245) the mid-gut forms in the female a well-developed 

 pouch-shaped stomach, which is sharply divided from the fore- and hind- 

 guts. The epithelium of the stomach is either itself glandular, oi' 

 there are special large glandular appendages. An intestinal muscula- 

 ture is either wanting or very slightly developed. 



The mid-gut of the Chcetognatha (Fig. 152, p. 227) takes a straight 

 course through the trunk cavity of the body, without lateral appendages. 

 A muscular layer is wanting. 



C. The Hind-gut and the Anus. 



The hind-gut comes from the proctodaeum of the larva, and forms 

 in the worms a tube, often very short, but generally clearly separated 

 from the mid-gut ; this tube opens externally through the' anus, and is 

 frequently called the reetum. The anus is either a separate aperture, 

 or else it unites with the apertures of other organs of the body. This 

 union is brought about through pit- or sac -like depressions of that 

 region of the body wall in which these apertures lie near each other ; 

 a cloaca is thus formed, which then opens externally by a new common 

 aperture, the cloacal aperture. 



The juxtaposition of the external apertures of inner organs and the invagination 



of the region common to these apertures are very frequent in the animal kingdom. 



We "will give only a few cases in illustration. In many Platodes the originally separate 



male and female genital apertures come to lie at the base of a common genital 



VOL. I P 



