166 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



which resembles blood in many respects but possesses no red 

 corpuscles. There are also certain " ductless glands " connected 

 with the lymphatic system, of which the largest is the spleen, a 

 reddish body attached to the bend and distal limb of the 

 stomach. 



The largest lymph-space is the body-cavity or ccelom, which 

 includes the abdominal and pericardial cavities. 



6. The respiratory organs of the dogfish are gills, adapted for 

 breathing the oxygen dissolved in the surrounding sea- water. 

 They consist of vascular folds arranged upon the posterior side 

 of the hyoid arch, and both sides of the first four branchial 

 arches. Water is taken in at the mouth and expelled through 

 the gill-slits to the exterior, so that a continual stream passes 

 over the gills, in the small vessels of which the blood is purified. 

 The haemoglobin of the red blood-corpuscles acts as an oxygen- 

 carrier, taking up a certain amount of free oxygen from the 

 exterior into a state of loose chemical combination, and parting 

 with it again to the tissues. 



A rudimentary gill (pseudobranch) is found on the anterior 

 wall of the spiracle. 



7. The excretory and reproductive organs are so closely con- 

 nected that it is best to consider them under the common heading 

 of urine-genital organs. 



The male dogfish possesses a pair of elongated narrow kidneys 

 extending nearly the whole length of the abdominal cavity and 

 situated close together below the vertebral column and above the 

 peritoneum. Each kidney is divided into a number of segments, 

 and its anterior half is distinguished as mesonephros (Wolffian 

 body) from its posterior half or metanepkros, the excretory pro- 

 ducts of these being carried off by distinct mesonephric (Wolffian) 

 ducts, and metanephric ducts (ureters). 



Each mesonephric duct, which, since it also acts as a spermi- 

 duct, may be termed urino-genital duct, is a convoluted tube 

 running along the ventral side of the corresponding mesonephros, 

 dilating into a vesicula seminalis, and finally opening into an urino- 

 genital sinus that communicates with the cloaca by a small aperture 

 placed on the end of a dorsally situated urino-genital papilla. 



The metanephric duct on each side is formed by the union of 

 several smaller ducts and opens into the dorsal side of the urino- 

 genital sinus. 



