THE ALIMENTARY CANAL. 59 



epithelial cells send processes outwards into the muscular coat. 

 The muscular coat is from 2.0^ to 25'^ thick, and towards the 

 anus has a strong sphincter, composed of from 20 to 30 

 circular fibres. Numerous muscle-fibres arise from the integu- 

 ment on either side, and pass inwards and downwards, to be 

 inserted into the peritoneal coat of the rectum by fine tendon- 

 like ends. All the muscle-fibres of the rectum resemble the 

 skeletal muscles of the imago, and each exhibits a close row of 

 central nuclei (see Histology and Histolysis of the Larval 

 Tissues), The limits of the distal intestine and the rectum are 

 not distinctly defined, but the cuticular intima of the latter 

 becomes thinner, and the epithelium exhibits a transitional 

 character towards the distal intestine. In the imago the two 

 are separated by a valve. 



The Tracheae of the Alimentary Canal, — The great lateral 

 tracheal trunks give off one or more branches on each side 

 in each segment to the alimentary canal. The largest branches 

 are those to the proximal intestine. Two very large trunks 

 join the intestine at the anterior part of the haemal flexure and 

 retain it in its place ; the other large tracheae enter the intes- 

 tinal coil and bind it together. These also send numerous 

 capillary branches to the Malpighian tubules. 



Intestinal Ganglia. — Viallanes [27, p, 74] describes a distinct 

 plexus of stellate ganglion cells in the muscular coat arranged in 

 four parallel rows, and extending the whole length of the 

 chyle stomach and intestines of the larva of Tipula gigantea. 

 I have been quite unable to find any such structure in the larva 

 of the Blow-fly, but no doubt ganglion cells derived from the 

 proventricular ganglion exist. 



Visceral Muscular Network. — Weismann [2] has described 

 a network of delicate muscle-fibres, which connect the walls of 

 the ahmentary canal with the alse of the dorsal vessel ; but 

 I have been unable to detect any such fibres, and, if they really 

 exist, they could scarcely fail to appear in some of my numerous 

 sections. 



The Lingual (Salivary) Glands are a pair of very large sac-like 

 glands, which consist of a thin cuticular membrane, covered 



