RiESPIRATORY ORGANS. 



199 



muscles. The pyloric caeca are tubular structures opening 



into the first part of the intestine just beyond the pylorus ; 



they vary in number from one to two hundred, and are very 



generally present. The liver is provided with a gall bladder 



which opens just beyond the pylorus. It generally contains 



much oily matter, but in some forms the oil occurs in all 



the tissues of the body and is not only a feature of the liver. 



The pancreas,* the presence of which in Teleostei used to be 



denied, is not usually a conspicuous 



structure, though functionally of great 



importance, especially in those Teleosts 



in which the stomach is without gastric 



glands. It is either embedded in the 



liver or diffused in the mesentery, and its 



duct opens in close connection with the 



hepatic duct. In some forms {Salmo, 



Gadus, Perca, Platessa, Brama, etc.) there 



is a small gland opening with the bile 



duct, which is possibly pancreatic. 



The Thyroid t body is represented by 

 small reddish masses lying ventral to the 

 ventral aorta, and the thymus f is placed 

 at the dorsal ends of the last pair of 

 branchial arches. 



The respiratory organs. — There is no 

 spiracle. The branchial apertures are 

 narrow slits and the tissue between them 

 has the form of an arch, not of a sep- 

 tum. In consequence of this the gills 

 themselves are filamentous, not lamellar 

 (Fig. 118). The external openings of the 

 clefts are covered by the oj)erculum. 

 The gill-filaments are borne in a double 



row (holobranch) by the four anterior branchial arches, the 

 last gill-aperture is smaller than the others, and the fifth 

 branchial arch never bears gill-filaments. 



Fig. 118. — Transverse sec- 

 tion through a branchial 

 arch of Zygaena (right 

 hand) and oJ Gadus (left) 

 (after E,. Hertwig, from 

 Wiedersheim) . a and v 

 afferent and efferent 

 blood vessels, 6 skeletal 

 branchial arch, hl^ and 

 6oZ-posterior and anterior 

 demibranch together con- 

 stituting a holobranch, 

 h septum between two 

 branchial apertures in 

 Zygaena, r cartilaginous 

 branchial ray supporting 

 the same, z small tooth- 

 like tubercle (sometimes 

 elongated as a gill-raker) 

 in a double row on the 

 branchial arch of Gadus. 



E . Lagviesse, Structvire, etc., du pancreas d'apres les travaux recents, 

 Journ. Anat. Phys., Paris, 30, 1894, p. 591 and 731. E. Goeppert, Die 

 Entwick. d. Pancreas Teleost., Morph. Jahrb., 20, 1893. 



t F. Maurer, Schilddriise u. Thymus in Teleostier, Morph. Jahrb., 

 xi., 1886, p. 128-75. 



