OESOPHAGUS, STOMACH, AND INTESTINE 335 



function may be : at any rate it is not a " lymphoid organ," and 

 the origin of the leucocytes occurring in it is not known. 1 



(ESOPHAGUS, STOMACH, AND INTESTINE. 



Ichthyopsida. The oesophagus in the Anamnia is short, and 

 usually not distinctly marked off from the stomach, though excep- 

 tions to this rule often occur (e.g. many Teleostei, Siren lacertina). 



The stomach is often defined as a widened section of the enteric 

 canal situated between the posterior end of the gullet and the 

 entrance of the bile duct. Such a dilatation can strictly only be 

 spoken of as a stomach when its epithelium possesses specific 

 characteristics and gives rise to gastric glands : in this sense 

 a stomach is wanting in Amphioxus, Cyclostomi, Holocephali, 

 certain Teleostei (e.g. Cyprinidae, certain Labridae, Gobiidse, and 

 Bleniidse, Syngnathus, Cobitis), and Dipnoi (Fig. 247). In many 

 Teleosts the bile-duct opens far forwards, so that the gastric 

 region is very short. 



In other Fishes, as well as in all Amphibians, a true stomach 

 is present, and is usually externally recognisable as a more or 

 less dilated sac ; it may be curved on itself, so as to form a 

 U-shaped loop, the two (cardiac and pyloric) limbs of which lie 

 parallel to one another (Fig. 244). In general, its form is adapted 

 to that of the body : thus Rays and Anurans possess a far wider 

 stomach than do most other Fishes and Amphibians (cf. Figs. 

 244-249). The stomach of Teleosts varies considerably in form. 2 



The intestine may be straight or nearly straight, or may be 

 more or less coiled, and in the former case a spiral fold or valve 

 may be developed in Fishes, to increase the absorptive surface. 



In the Lamprey a longitudinal fold or typhlosole, taking a slightly 

 spiral course, extends into the lumen of the intestine. In Elasmo- 

 branchs, Ganoids, and Dipnoans, the fold is more highly developed 

 and forms a well-marked spiral valve, the turns of which may lie 

 so close together as to almost fill the cavity of the intestine (Fig. 

 244). 3 In the Ganoids it begins to undergo degeneration : thus 

 in Lepidosteus it is only present in the hinder part of the in- 

 testine (Fig. 245). Traces of a spiral valve can even be recognised 

 amongst the Teleostei (Cheirocentrus). 



1 The so-called " Hassal's corpuscles " in the thymus arise secondarily from 

 groups of the small epithelial cells. 



- In numerous Teleosts (e.g. Tinea vulgaris, Cobitis fossilis) outer longi- 

 tudinal and inner circular striated fibres are present in both stomach and intestine 

 externally to the unstriated muscular coat. They grow backwards from the 

 oesophagus. 



3 The arrangement and extent of the valve vary considerably ; it may begin 

 close behind the pylorus, or the valveless anterior part (" bursa entiana") maybe 

 relatively longer. In some cases the valve is scroll-like, and not spiral (e.g. 

 Carcharias). 



