THE BLOOD-SYSTEM. 57 



nopleure, which forms the enderon of the alimentary canal, 

 and the somatopleure^ which constitutes the enderon of the 

 integument. The terminations of the water- vessels open into 

 this cavity. In Annelids, there is a similar perivisceral cavity 

 communicating in the same way with the segmental organs ; 

 but, in most, there is, in addition, a system of canals with 

 contractile walls, Mdiich, in some, communicate freely with 

 the perivisceral cavity, but, in the majority, are shut oif from 

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

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

 pseud-haemal system. The fluid which occupies the perivis- 

 ceral cavity contains nucleated corpuscles, and has the 

 characters of ordinary blood. It seems probable that the 

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

 resembling hasmoglobin, represents a sort of respiratory 

 bloocl. 



In the Arthropoda^ no segmental organs or pseud-hcemal 

 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 float in their fluid con- 

 tents. In the higher forms, a valvular heart, with arteries 

 and capillaries, appears, but the venous system remains more 

 or less lacunar. In the 3IoUusca^ the same gradual differen- 

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

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

 the exterior by the " organs of Bojanus " — which resemble 

 V3ry simple segmental organs, and appear to be always asso- 

 ciated with the renal apparatus. 



In the Vertebrata, AmiyJiloxus 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 ve)iosus, atriutn, ventricle)^ arteries, capil- 

 laries, and veins, and a system of lymphatic vessels connected 

 with the veins. The lymphatic fluid consists of a colorless 

 plasma, with equally colorless nucleated corpuscles ; the blood- 

 plasma contains, in addition, red corpuscles, which are nucle- 

 ated in Ichthyopslda and ^auropsida, but have no nucleus 

 in the 3Iamm(dia. The lymphatic vessels always communi- 

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

 lower Vertebrates are themselves, to a great extent, irregular 

 sinuses. The venous system presents many large sinuses in 

 the lower Vertebrates ; while, in the higher forms, these 

 sinuses are for the most part replaced by definite vessels with 

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



