XVI ASTERIAS — "BLOOD-VESSELS" GENITAL STOLON 45 I 



altered into tough fibres, but a portion of each strand remains 

 living, and in this way the connective tissue is formed. In the 

 interstices of the network of fibres a semi-fluid substance (the 

 unaltered jelly) is found, and this is traversed by free, wandering 

 amoebocytes. In other places the jelly becomes more fluid and 

 forms tlie plasma, or liquid of the blood, whilst the amoebocytes 

 form the blood corpuscles. The blood system thus arises from 

 regions of the archicoel where fibres are not precipitated. 



Now in the Starfish the whole substance of the body-wall inter- 

 vening between the ectoderm and the coelomic epithelium really 

 represents the archicoel. The formation of fibres has, it is true, 

 proceeded to a certain extent, since there are interlacing bundles 

 of these, but there are left wide meshes in which amoebocytes 

 can still move freely. Apart from the skeleton, therefore, the 

 tissues of the body-wall of the Starfish do not exhibit much 

 advance on those of a Jellyfish. If anything is to be compared 

 to the blood system of the higher animals it must be these 

 meshes in the connective tissue. From observations made on 

 other Echinoderms it appears probable that the colour of the 

 skin is due to amoebocytes loaded with pigment wandering 

 outwards through the jelly of the body -wall and disintegrating 

 there. The strands regarded as blood-vessels by Ludwig are 

 specially modified tracts of connective tissue in which fibres are 

 sparse, and in which there are large quantities of amoebocytes 

 and in which the " jelly " stains easily. Cuenot ^ suggests that 

 they are placed where new amoebocytes are formed ; this is quite 

 possible, and in this case they ought to be compared to the 

 spleen and other lymphatic organs of Vertebrates, and not to 

 the blood-vessels." 



The organ regarded as the heart, however, belongs to a 

 different category : it is really the original seat of the genital 

 cells and should be termed the " genital stolon." Careful sections 

 show that at its upper end it is continuous with a strand of 

 primitive germ-cells which lies inside the so-called aboral blood- 



^ "Cont. a rfitude anat. des Asturides," Arch. Zool. Exp. (2) v. his, 1887, p. 104. 



"^ The analogy of Echinoidea (see p. 529) might suggest tliat, like the lacteals in 

 man, these strands were channels along which the products of digestion diffused 

 outward. No connexion, liowever, between the oral ring and the alimentary- 

 canal has been made out, nor do there a])pear to be such strands developed in the 

 proximity of the wall of the digestive tube. A connexion between the aboral ring 

 and the rectum through a mesenteric cord has been asserted, but this is doubtful. 



