450 ECHINODERMATA ASTEROIDEA chap. 



joined at its upper end a circular cord of pigmented material 

 adhering to the dorsal wall of the coelom (lying in fact within 

 the aboral sinus), from which branches could be traced to the 

 generative organs. Ludwig concluded that he had at last dis- 

 covered the true blood-vessels, though the facts that the radial 

 strands and the oral circular strand absorbed neutral carmine 

 strongly and that the vertical and aboral strands were pig- 

 mented, constituted a very slender basis on which to found such 

 a conclusion. The colour apparently appealed to the imagination, 

 and it is undoubtedly true that the " plasma " or blood-fluid of 

 other animals often absorbs stain strongly. 



The strands were accordingly named "radial blood-vessel s," 

 " oral blood-ring," " aboral blood-ring " ; and the brown vertical 

 strand was called the " heart," although no circulation or pulsa- 

 tions had ever been observed. When later investigations 

 revealed the fact that the so-called heart was practically solid, 

 the term "central blood -plexus" was substituted for heart, 

 although it was still regarded as the central organ of the system. 

 The name " perihaemal " was given to the spaces so called because 

 they surrounded the supposed blood-vessels. 



In order to come to a satisfactory conclusion on the matter 

 some general idea as to the fundamental nature and function of 

 the blood-vessels in general must be arrived at. Investigations 

 made on various groups of animals, such as Annelida, Mollusca, 

 Crustacea, Vertebrata, show that at an early period of develop- 

 ment a considerable space intervenes between the alimentary canal 

 and the ectoderm, which is filled with a more or less fluid jelly. 

 Into this cavity, the so-called " primary body -cavity " or " archi- 

 coel," amoebocytes, budded from the ectoderm or endoderm or both, 

 penetrate. In this jelly with its contained amoebocytes is to be 

 found the common rudiment both of the connective tissue and of 

 the blood system. The resemblance of the archicoele and its 

 contents to the jelly of a Medusa is too obvious to require special 

 insistence on, and therefore in the Coelenterata it may be stated 

 that there is to be found a tissue which is neither blood system 

 .nor connective tissue but is the forerunner of both. 



In the higher animals as development proceeds the jelly 

 undergoes differentiation, for some of the amoebocytes become 

 stationary and connected with their pseudopodia so as to form 

 a protoplasmic network. A portion of this network becomes 



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