SCYLLIUM CANICULA 233 



folds are generally spoken of as mesenteries, and they give rise to a 

 series of lateral peritoneal folds which hold the other viscera in 

 position ; such side folds are termed omenta and receive their name 

 from the organs they surround. Thus we can recognise a gastro- 

 hepatic omentum connecting liver and stomach, in this run the bile 

 duct and portal vein ; a gastrointestinal omentum, binding stomach 

 and intestine ; and a gastro-splenic omentum, joining the spleen to 

 the stomach. 



Respiratory System. 



The anatomy of the respiratory system has already been 

 considered in dealing with the alimentary canal, since the respiratory 

 organs, the gills, are derived from modified outgrowths of the walls 

 of the pharynx. All the blood leaving the heart is taken to the gills 

 by a series of vessels, collected up by another set and then conveyed 

 all over the body. In the tiny filaments the vessels break up into 

 numerous small capillaries, thus allowing the blood to be brought 

 quite close to the surrounding water. This water is constantly 

 being changed by the action of the branchial muscles, and so the 

 oxygen contained in it is able to pass into the blood, and at the same 

 time the carbon dioxide in the latter can pass out into the water by 

 diffusion. Unlike the frog, respiration is limited to this region and 

 not carried on over the general surface of the body. 



Circulatory System. 



The circulatory system in Scyllium is in some ways similar 

 to that of Rana, but it is of a more primitive type and is of interest in 

 comparative morphology, as the main outlines of its general plan are 

 encountered in the embryos of all the higher Craniates. It is com- 

 posed of a blood-vascular system and a lymphatic system. The 

 blood- vascular system is a closed one as in all Craniates, and consists 

 of a heart, arteries, capillaries and veins which form one series of 

 closed vessels not communicating with the outside. As "in the frog, 

 some of the fluid plasma of the blood oozes through the walls of 

 the capillaries to bathe the various tissues, and this is collected up 

 by the lymphatic vessels and returned to the blood again. Owing 

 to the difficulty of finding the lymphatic vessels, particularly in 

 Scyllium, by the ordinary methods of dissection the term " circulatory 

 system " is often employed to mean the blood- vascular system only. 



Heart. 



The heart is a stout muscular organ the rhythmic 

 contraction of whose walls produces the difference in pressure 

 necessary to keep the blood circulating, and so it forms the centre 



