(ESOPHAGUS, STOMACH, AND INTESTINE 257 



In Fishes and Dipnoans the thymus is more or less subdivided, 

 and is -situated dorsally to the gill-arches. In Amphibians it 

 lies behind and above the articulation of the lower jaw, and in 

 Keptiles in the neighbourhood of the carotid artery, either close in 

 front of the heart {e.g., Snakes) or more anterioily. In Birds, as 

 in young Crocodiles, it is elongated and more or less lobed, 

 extending all along the neck (Fig. 205). In Mammals the greater 

 part of the thymus is situated in the thorax, between the sternum 

 and heart, only a small portion extending into the neck. It is 

 largest in young animals, and usually becomes more or less 

 completely degenerated subsequently. 



(ESOPHAGUS, STOMACH, AND INTESTINE. 



Ichthyopsida. — The oesophagus is short, and usually not 

 distinctly marked off from the stomach, though exceptions to this 

 rule often occur {e.g., many Teleostei, Siren lacertina Fig. 210, A). 

 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 cannot 

 accurately be spoken of as a stomach unless its epithelium 

 •possesses specific characteristics and gives rise to gastric glands 

 (p. 267) ; in this sense a stomach is wanting in Amphioxus, Cyclo- 

 stomi, Holocephali, certain Teleostei {e.g., Cyprinidse, certain 

 Labridse, Gobiidae, Bleniidse, Syngnathus acus, Cobitis fossilis), 

 and Dipnoi (Fig. 209). Whether this is a primitive character in. 

 these forms or is due to degeneration is uncertain. 



In other Fishes (Elasmobranchs, Ganoids, numerous Teleosts), 

 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. 

 206). 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 (comp. Figs. 206 — 210), and this rule 

 holds good also for Eeptiles. The stomach of Teleosts varies con- 

 siderably in form.i 



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- 



' 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 

 (jesophagus. 



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