Boyal Microscojpical Society. 59 



These differ from the primary vesicles in opening into each other, 

 and into the primary vesicles, just as the latter open into the trans- 

 verse vesicles. 



These secondary and tertiary vesicles give off minute dichoto- 

 mously dividing vessels, which dip into the muscular layer, and 

 ramify in and over all the organs of the body, forming a true 

 pseudo-haemal system. The main trunks, lateral branches, and 

 vesicles of the water vascular system have an external coat of fine 

 muscular fibres, an inner membranous coat, and an epithelial lining. 

 The pseudo-haemal vessels seem to consist of membrane only. 



The whole water vascular system is filled with a viscid mole- 

 cular fluid, and is most conveniently seen by staining the tissues 

 with an alkaline solution of carmine, or by treating a portion of the 

 worm with dilute acetic acid. Specimens so treated and mounted 

 in glycerine jelly are exhibited under the microscopes. 



I will recapitulate the main points, in order to avoid any chance 

 of being misinterpreted. The water vascular system consists, 

 according to my observations, of a ring round the pharynx, con- 

 nected with pores in the outer sides of the hps, and giving off a 

 plexus of vessels posteriorly. From this plexus two large trunks, 

 one dorsal and the other ventral, pass to the posterior extremity of 

 the body, and two lateral sets of smaller longitudinal vessels accom- 

 pany the lateral bands. 



Both lateral, dorsal, and ventral vessels give off transverse vessels, 

 which form a plexus, and bear numerous vesicles. The vesicles 

 open into other, and these into other vesicles, the latter giving 

 off fine vessels, which I have called pseudo-haemal. The pseudo- 

 haemal vessels ramify in and upon all the other tissues. 



The similarity of the whole water vascular system of Ascaris to 

 the ambulacral system of the Echinodermata can hardly escape 

 notice. If the Ascaris possessed ambulacral processes in connection 

 with the vesicles above described, the two sets of organs would be 

 nearly identical. 



The water vascular system of Ascaris seems also to afford a strong 

 indication that Professor Huxley's hypothesis concerning the nature 

 of the so-called pseudo-haemal system of Echinoderms and Annelida 

 is correct, that they really belong to the same system of tubes with 

 the ambulacral and water vascular systems. 



I further suspect very strongly that in Ascaris we see a transi- 

 tional form, in which the water vascular system is immediately con- 

 cerned with the function of nutrition, with a somewhat rudimentary 

 condition of the alimentary canal, seen in the absence of a muscular 

 coat beyond the pharynx, which is connected closely with the con- 

 dition of things met with in the anenterhelminths, or worms without 

 any intestinal canal, where, as in Tenia, the function of nutrition 

 undoubtedly devolves upon a system of tubes homologous with a 



VOL. V. F 



