. Elements of Comparative Anatomy. 162 COMPAEATIVE ANATOMY. position of the anus, while tlie SipunculidiB seem to differ more from tliem. But this position of the anus is really only a further development of that dorsal posi- tion which it has in many of the Vermes, and does not in any way affect the homology of their enteron with that of the other members of this group. § 132. The metamerism of the body in the Annulata affects the en- teric tube; but there are various other differentiations in it, which are due to adaptations to special modes of life. Here again the enteric canal begins as a
. Elements of Comparative Anatomy. 162 COMPAEATIVE ANATOMY. position of the anus, while tlie SipunculidiB seem to differ more from tliem. But this position of the anus is really only a further development of that dorsal posi- tion which it has in many of the Vermes, and does not in any way affect the homology of their enteron with that of the other members of this group. § 132. The metamerism of the body in the Annulata affects the en- teric tube; but there are various other differentiations in it, which are due to adaptations to special modes of life. Here again the enteric canal begins as a caecal invagination. The aproctous condition, which persists in most of the Platyhelminthes,is passed through by these forms at an early stage in development. The enti'ance to the fore-gut is most variously differentiated in the Hirudinea. In some the pro- tractile oesophagus is greatly complicated, in others its en- Fig. IS. En- teric canal of Sanguisuga. 0 ffisopliagus. c Posterior pair of casca. a Fig. '74. Enteric canal of Aphrodite, o An- terior jDortion. h Middle (muscular) portion of the fore-gut. c Branched csecal appendages of the mid-gut. a Anus. trance is armed with chitinous ridges, which are the first signs of jaws. But in most the mid-gut is beset by pouch-like diverticula (Fig. 73), which are branched in Clepsine; the last two of these diverticula sometimes form longer csecal tubes (c) on the narrow hind-gut, which extends to the end of the body (Clepsine, Haemopis). These are the only cseca of the gut of Aulacostomum. In others the cseca are merely indicated by constric- tions. In all cases these arrangements correspond to the metamerism expressed also in the nerve- chord. In nearly all Annelids the fore-gut is separated into several, often very different, portions. A median portion is distinguishable by its more powerful muscular investment, and is separated from the mid-gut by a tract of varying length. Among the Scoleina, this portion, which is knoWn
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