434 INTRODUCTION TO CRANIATA [CH. 



haemoglobin. The carbon dioxide is set free in the respiratory 

 organs. 



On page 202 it was pointed out that both blood and connective 

 tissue have been derived from a jelly-like secretion such as is found 

 in Coelenterata. This in the embryo coelomate animal fills up the 

 interstices between ectoderm, endoderm and coelomic sacs, these 

 interstices being collectively termed the primary body-cavity or 

 haemacoel. It was also pointed out there, that whereas in that 

 part of the jelly which was converted into connective tissue a large 

 number of fibres were developed, in the portion destined to form 

 blood, on the contrary, no fibres appeared and the jelly remained 

 fluid, and in consequence the amoebocytes which had wandered into 

 it from the neighbouring epithelia were able freely to move about. 

 In Annelida, Arthropoda and Mollusca certain of the blood-spaces 

 acquire muscular walls derived from the adjacent coelomic sacs, 

 and thereby attain contractility which may be specially localised in 

 a dilatation called the heart. The spaces with muscular walls are 

 the arteries. In Craniata a further differentiation has taken place : 

 we find not only a definite heart and arteries leading away from it, 

 but also equally definite veins leading into it as described above, 

 and arteries and veins are connected with one another by narrow 

 channels called capillaries with well-marked walls. Heart, 

 arteries, veins and capillaries are all lined by a single layer of 

 flattened cells called an endothelium, which has been developed 

 from the flattening out and union of a certain number of amoebo- 

 cytes. The capillaries possess no other wall, but arteries and veins 

 have outside this a wall of elastic and fibrous connective tissue in 

 which is embedded a zone of circular muscle-fibres. These structures 

 are all derived from the adjacent coelomic sacs." The muscles of 

 blood-vessels do not contract rhythmically and spontaneously like 

 those of the heart, but are in a state of continued contraction called 

 tone. This tone is under the control of the nervous system through 

 the medium of special "vasomotor" fibres, and thus the supply of 

 blood to an organ can be varied according to its need. 



In Craniata however, outside the definite arteries, veins and 

 capillaries, there exists a large portion of the haemocoel in the 

 form of irregular channels and interstices, in many cases without 

 definite walls, an endothelium being found only in the larger trunks. 

 This system of spaces is known as the lymphatic system. It 

 contains a clear fluid in which amoebocytes float, but no haemo- 

 .globin-coiLtaining cells, and at one or several points the main 



