228 COMPARATIVE ANATOMY. 



In post-embryonic time the organ always shows a lymphoid 

 structure, and, on account of its richness in white blood-corpuscles, 

 certainly has important physiological relations to the organism as a 

 whole. This is probably especially the case in Mammals, as it 

 here attains a large development, extending in the embryo back- 

 wards from the region of the larynx above the sternum as far as 

 the diaphragm. Later it undergoes a retrogressive metamorphosis, 

 and finally becomes more or less completely obliterated, though it 

 persists for a considerable time in Man, often for many years after 

 birth. In all other Vertebrates it persists throughout life, and lies 

 as a lobulated or cord-like organ in the anterior or lateral region 

 of the neck; thus in bony Fishes, for instance, it is situated 

 behind the gills in the neighbourhood of the fibrous band which 

 connects the gill-cover with the pectoral arch, and in Amphibians 

 it lies behind and above the articulation of the lower jaw (comp. 

 also Fig. 185, Tm). 



II. OESOPHAGUS, STOMACH, AND INTESTINE. 



Fishes and Amphibia. While in Amphioxus a widened 

 section of the alimentary canal is probably to be looked upon as a 

 sort of stomach, in Cyclostomi, Dipnoi, Chimserse, certain 

 Teleosteans, and many branchiate Amphibians, a stomach is 

 not plainly marked off from the rest of the gut, which usually has 

 a more or less straight course. In this case the only externally 

 visible boundary between the stomach and intestine is, as already 

 mentioned, the point where the bile-duct (ductus choledochus) 

 perforates the wall of the latter. In other Fishes, as, for instance, 

 in Squalidae, all Ganoidei, numerous Teleostei, as well as in 

 theDerotremata, Myctodera, and Anur a, the stomach appears 

 more or less dilated and sac-like ; it may also be curved on itself, 

 so that one can distinguish between a part running backwards 

 (descending portion) (Fig. 186, M), and another extending forwards 

 (ascending portion), the two lying parallel to one another (PJK). In 

 general, it becomes adapted to the form of the body : thus Rays and 

 Anurans possess a far wider stomach than do most other Fishes and 

 Amphibians (cp. Figs. 189 and 190) and this rule holds good also for 

 Reptiles. The stomach of Teleosteans varies considerably in form. 



The oesophagus is short, and usually not distinctly marked 

 off from the stomach, though exceptions to this often occur, as, 

 for instance, in many Teleostei, and in Siren lacertina amongst 

 the Amphibia (Fig. 189, Oe). 



A longitudinal fold extending into the lumen of the intestine, 

 the first traces of which are seen in Ammocoetes, is to be looked 

 upon as a structure designed for increasing the digestive surface : 

 this is also present inElasmobranchs, Dipnoj., and Ganoidei, 



