60 COMPAEATIVE ANATOMY. 



i) Vascular System. 

 §43. 



The substances prepared by the digestive process for tlie 

 nourishment of the body arOj in the lowest organisms which take 

 in solid food, merely distributed from the digestive spaces into the 

 protoplasm of the body. When a distinct digestive tube is formed, 

 nutriment passes through its walls straight into the parenchyma of 

 the body, so that the mesoderm and ectoderm, with the organs 

 differentiated from them, are nourished by the endoderm. This is 

 characteristic of the Coelenterata and some groups of Vermes. In 

 many others a dividing of the mesoderm occurs, which takes the 

 form either of canalicular cavities, or of a complete splitting of the 

 mesoderm into an outer plate attached to the ectoderm, and an 

 inner one attached to the endoderm. Between these dermal and 

 gastric layers of the mesoderm is the body-cavity, or perienteric- 

 cavity (coelom), in which a fluid, to be regarded as the nutrient 

 fluid, is collected. When morphological elements are found in this 

 fluid, they are derived from the cells of the mesoderm. This fluid 

 is not at first exclusively nutrient ; it also subserves locomotion, by 

 swelling out different parts of the body at the will of the animal. 

 An important function of this kind is also played by the water, which 

 in many cases is taken into the coelom from the exterior. 



The movement of the fluid in the general cavity of the coelom is 

 at first effected by the movements of the body. Contractions and 

 expansions of the body-wall cause the fluid which is shut in by the 

 dermo-muscular tube to continually change its position ; this may 

 be regarded as the lowest form of a circulation of the blood. 

 In this case the passages have not special walls, nor are there any 

 special ari-angements for regulating the circulation. 



This simple condition persists in many divisions in which the 

 coelom is developed (Bryozoa); in others canalicular cavities arise, 

 which are arranged regularly, and have the form of vessels, and 

 may undergo further complications. Their contents form the hsemal 

 fluid or blood (Nemertiues). When in addition to these vessels 

 a perienteric-cavity is formed, the vascular system, which is partly 

 enclosed in it, is either completely shut off from it (many Annelida), 

 or is placed freely in communication with it at one or more points 

 (Mollusca, Arthropoda, Vertebrata). In the latter case the 

 vascular cavities must have arisen as portions of the body-cavity, 

 while in the former case the body- cavity was not formed untfl after 

 the vessels. The formation of the body-cavity is therefore, in the 

 case exempUfied by the Annelids, to be regarded as a secondary 

 process ; and the formation of a hollow space in the mesoderm has 

 accordingly led to two different results successively; on the first 

 occasion to the formation of blood vessels, on the second occasion to 

 the formation of a body-cavity. 



