TIII: in - , I:M. 57 



nopleure, which forms the enderon -.1 th- alimentary canal, 

 and the somatopleure, which constitutes if,,, aodtv 

 integument. The termiaationi of the water-vessels .. 

 this civily. In Annelids, there is a similar periviaceral c;t 

 communicating in the same way with the segmental organ* ; 

 but, in most, there is, in addition, a syst< i iaU with 



contractile walls, which, in some, communicate freely \\itli 

 tlie perivisceral cavity, but, in the majority, are shut <>tl fr<m 

 it. These canals are filled by a clear, usually non-corpuscu- 

 lated fluid, which may be red or green, and constitute the 

 pseud-hcemal system. The fluid wmob oeouplM tin- p : 

 coral cavity contains nucleated corpuscles, and has 

 characters of ordinary blood. It seen -able that the 



fluid of the pseud-haemal vessels, as it contains a substance 

 resembling haemoglobin, represents a sort of respiratory 

 blood. 



In the Arthropoda, no segmental organs or pseud-haemal 

 vessels are known. In the lowest forms, the perivisceral 

 cavity and the interstices of the tissues represent the whole 

 blood-system, and colorless blood-cells Hoat in their fluid 

 tents. In the higher forms, a valvular In-art, with arteries 

 and capillaries, appears, but the venous system H-main* more 

 or less lacunar. In the Mollusca, the same gradual ditL r. n- 

 tiation of the blood-vascular system is observable. In very 

 many, if not all, the blood-cavities communicate directly with 

 tha exterior by the " orgaps of Bojanus " which resemble 

 very simple segmental organs, and appear to be always asso- 

 ciated with the renal apparatus. 



In the Vertebra ta, Amphioxus has a system of blood-ves- 

 sels, with contractile walls, and no distinct heart. In all 

 the other Vertebrates there is a heart with at fewest three 

 chambers (sinus venosus, atrium, ventricle), arteries, capil- 

 laries, and veins, and a system of lymphatic vessels c< 

 with the veins. The lymphatic fluid consists of a < 

 plasma, with equally colorless nucleated corpuscles : the blood- 

 plasma contains, in addition, red corpuscles, \vhich ;ire n 

 ated in Ichthyopsida and S<ui /-/<x *'</<>, Imt have no nu- 

 in the Mammli<t. The lymphatic vessels always communi- 

 cate with the interstitial lacuniv of the tissues, and in the 

 lower Vertebrates are themselves, to a ;j- rular 



sinuses. The venous system presents many lani'' sinuses in 

 the lower Vertebrates; while, in the higher hese 



sinuses are for the most part replaced by definite vessels with 

 muscular walls. But the " serous cavities " remain as vmst 



