211 



elliptical or circular red blood corpuscles are impregnated 

 with haemoglobin, the oxygen - carrying pigment ; the 

 amoeboid colourless leucocytes act as phagocytes, and de- 

 stroy injurious micro-organisms which enter the blood 

 stream. 



In Vertebrates the blood is not in direct contact with 

 the body tissues ; the oxygen and nutritive plasma (exuded 

 through the capillary walls) and the waste products are 

 interchanged through the intervening lymph, which bathes 

 the tissues and forms the lymphatic system. The larger 

 lymphatic ducts open into the large veins ; and the numerous 

 lymphatic " glands " supply the amoebocytes of the lymph 

 and blood. 



Write an Account of the Respiratory Organs of. Craniates. 



Cyclostomata, Fishes, all young and certain adult Am- 

 phibians have gills for breathing in water. The portions of 

 the pharynx wall between the gill-clefts form partitions or 

 septa ; and the mucous lining of the septa is raised into 

 vascular folds which are the internal gills. In Teleosteans 

 the septa are reduced, being merely narrow strips containing 

 the supporting cartilaginous visceral arches ; and the gills 

 are double rows of filaments, each supported by a car- 

 tilaginous gill-ray and projecting from the arch. In Elasmo- 

 branchs the septa are wide, and form complete partitions 

 between the gill-clefts (elasmobranch) ; and the gills, being 

 attached to the septum along almost their entire length, 

 are lamelliform, and have internal supporting rays. 



The blood is spread out on the gills and there oxygenated. 

 In the Dogfish, Skate, and other Elasmobranchs, a flap of 

 integument extends back from the front edge of each 

 cleft and overlaps its external opening ; this flap acts like 

 a valve, it allows outflow of the water from the pharynx, 

 but prevents inflow. In higher fishes (e.g., Teleosteans and 

 Dipnoi) the flap of the front cleft becomes a large operculum 

 overlapping all the clefts. 



Generally there are six visceral clefts ; the first one 

 (in front of the hyoid arch) is the modified spiracle, present 

 only in Elasmobranchs and certain Ganoids, and it contains 

 a vestigial gill, the pseudobranch. The last gill-cleft has 

 only a half-gill (hemibranch) on its anterior wall. The last 

 branchial arch bears no gills. In Teleosteans, only the first 

 four branchial arches bear gills. 



The external gills (usually temporary organs) of Cross- 

 opterygian Fishes, Dipnoi, and Amphibians have a different 



