70 LECTURE VI. 



The oesophagus (fig- 31, i) is muscular, and four or five lines in 

 length, narrow, slightly dilated posteriorly, and attached to the mus- 

 cular parietes by radiated filaments. Its cavity is occupied by three 

 longitudinal ridges, which meet in the centre of the canal. It is 

 separated by a well-marked constriction from the second part of the 

 alimentary tube (c, c), which extends to the terminal outlet (<7), without 

 presenting any natural division into stomach and intestine. The lower 

 third of the tube is the widest. Numerous long pyriform villi project 

 from the mucous lining of the alimentary canal. Many minute filaments 

 pass from the intestine to the soft obtuse papillae which project from the 

 walls of the abdomen into that cavity, and which are called " the nutri- 

 tious appendages" by Cloquet. The nutriment which these processes 

 or appendages are presumed to imbibe, is collected^ according to the 

 same author, into two canals, situated each in a narrow tract of 

 opaque substance, which extends along the sides of the body, and 

 has sometimes been mistaken for a nerve, and which Vallisnieri 

 believed to be a trachea. Morren has lately described and figured 

 the nutritive appendages as hollow vesicles : he calls them " Vesicules 

 aeriennes" because, he says, " they evidently subserve respiration by 

 furnishing air to the blood." Few physiologists are likely to acquiesce 

 in this view, which makes the respiratory apparatus of an animal 

 having no other atmosphere than the mephitic gases of the intestinal 

 tube, the largest and most extensively developed organ in the whole 

 body. 



With reference to the organisation of the Nematoid Entozoa, not 

 j)arasites of the human subject, I shall limit my remarks to those 

 structures which offer interesting approximations and analogies to the 

 organisation of higher vermiform animals, and of the existence of 

 which we must have remained ignorant if our attention had been 

 wholly confined to the human Entozoa. I may first refer to a 

 secreting apparatus, consisting of four slender blind tubes, each about 

 two lines in length, which are placed at equal distances around the 

 commencement of the alimentary canal in the Gnathostortia spinigerum, 

 a small nematoid worm closely allied to Strongylus, which I dis- 

 covered in the tunics of the stomach of a tiger.* The mouth of this 

 Entozoon is a vertical fissure, bounded on each side by a jaw-like lip, 

 the anterior margin of which is produced in the form of three straight 

 horny points. The secerning tubes terminate at the mouth by their 

 smaller extremities, and there pour out a semi-pellucid secretion. 

 They are analogous to the similarly simple salivary cEeca in the 

 Holothuria ; and their coexistence with a structure of the mouth, 



* Proceedings of the Zoological Society, November, 1836. 



