ALIMENTAEY CANAL OF VERMES. 



157 



■y 



■ee. 



\vi- 



it is placed, as a rule, at the hinder part of the body, aud is some- 

 times ventral and sometimes dorsal. 



A differentiation of the enteric tube into several functionally 

 different portions can always be made out ; accessory organs for the 

 prehension of food are also often present at 

 the entrance into the digestive cavity. The 

 three portions, which are here present for the 

 first time, are distinguished as fore-, mid-, 

 and hind-gut; the last is absent when there 

 is no anus. 



The primitive form of enteron agrees with 

 the characters which are seen in the Gastrula- 

 form (§ 28). It makes its appearance in all in 

 the embryonic commencement of the organism 

 as a ctecal cavity, with but slight complica- 

 tions, which opens on the surface at one point 

 only; it persists in this form in the lower 

 Vermes. This opening serves for the inges- ^t^^ 



tion of food, and also for the ejection of its 

 undigested remains ; it is mouth and anus at 

 the same time. This arrangement is very 

 common among the Platyhelminthes, being 

 the only condition of the alimentary canal 

 among the Trematoda, and the dominant one 

 among the Turbellaria. In the rhabdocoelous 

 Turbellaria the alimentary canal is distinctly 

 marked in its anterior portion only, and has 

 the form of a simple blind tube extending 

 through the body. The simple mouth varies 

 in position ; it may be in the anterior portion 

 of the body, or towards the middle of the 

 ventral surface, and lastly, even in the pos- 

 terior portion; it leads into a muscular 

 pharynx, which is seldom absent (Schizos- 

 tome£e), and which is, in many cases, pro- 

 tractile. This is the portion of the alimentary 

 tract, which, under many modifications, can 

 be most clearly traced in most divisions of the 

 Vermes. 



§ 128. 



Fig. 67. Prorhynclius 

 fluviatilis. o Mouth, 

 oe Qilsophagus, protrac- 

 tile like a proboscis. 

 i Enterou. (jl Glands 

 opening into the enteron. 

 c Ciliated pits, x Spike 

 in the organ above the 

 cesophagas, which ends 

 caecally at y. ov Ovary, 

 in which there are, in 

 the anterior parts, ova 

 at various stages of 

 development. 



In the dendi'ocoelous Turbellaria the gut 

 is adapted to the broad form of the body. 



The mouth is (Fig. 68, o) placed ventrally, and often near the 

 middle. The muscular pharynx (^j) is often metamorphosed into 

 a proboscidiform organ, which is capable of great enlargement in 

 size, and is cylindrical or drawn out into lobes. It leads into an 

 enteric cavity (;;), which occupies the middle of the fiat body, and 

 which is broken up into numerous branches, which pass towards 



