iv VERMES INTESTINAL CANAL 201 



The hind-gut, mostly short, often hardly distinguishable, comes from 

 the proctodseum of the larva or embryo ; its epithelium is derived 

 from the ectoderm. It ejects the undigestible remains of food through 

 the anal aperture. We will treat of these three divisions separately, 

 taking the worms in order. 



A. The Fore-gut. 



The fore-gut is called the gullet or oesophagus in the Nemertina. 

 It is chiefly to be distinguished from the mid-gut which follows it by 

 the finer structure of its walls. Here and there glands have been 

 observed entering it ; these are salivary glands. There are no special 

 muscular swellings in the oesophagus of the Nemertina, a want which 

 is compensated for by the development of a special proboscidal 

 apparatus. 



The oesophagus of the Nematoda is always distinctly separated 

 from the mid-gut, and lined by a strong cuticle, a continuation of the 

 cuticle of the outer integument. Its muscular wall is always 

 thickened into a generally round or egg-shaped pharynx, which 

 consists principally of muscle fibres placed radially to its axis. The 

 mouth often lies at the base of a variously shaped buccal cavity, 

 provided with hard teeth, lips, papillae, etc., and the pharynx itself may 

 fall into two consecutive parts by means of a transverse constriction. 

 Less frequently glandular tubes entering the buccal cavity have been 

 observed. 



The fore-gut of the Annulata shows very various adaptations. In 

 the Hirudinea we can already distinguish two types. In the 

 Rhynchobdellidce a pharyngeal apparatus is developed, which agrees 

 even in details with that of the Tridada, Alloioccela, and many 

 Polyclada among the Platodes. The mouth leads into a cylindrical 

 pharyngeal pouch proceeding backwards, at whose posterior end a 

 muscular cylindrical tube, the pharynx, rises, and projects freely 

 forwards into the pharyngeal pouch. We have then to use the 

 terminology adopted in connection with the Platodes a tubular 

 pharynx plicatus, which is protruded (not evaginated) from the oral 

 aperture. In the Gnathobdellida', on the contrary, the muscular Aval! 

 of the oesophagus itself is thickened and projects into the lumen, 

 generally in the form of three longitudinal folds or ridges. These 

 ridges are often very strongly developed (jaAvs), and at their anterior 

 projecting edges they are finely and sharply toothed (jaAV teeth). 

 The use made of jaw and teeth by the medicinal leech is Avell known. 

 From the Avail of the pharynx many strong muscle fibres radiate out 

 to the body Avail. The ducts of Avell-developed salivary glands enter 

 into the pharynx. 



^Eolosoma among the Oligochceta shoAvs a very simple arrange- 



