1898] -■ / XX I T LA TE A XC EST 11 Y OF THE VER TEBRA TA 2 5 



muscular region, modifications would have been taking place in 

 the ventral vegetative region, always in adaptation to the new 

 functions of the alimentary canal. This canal, besides being greatly 

 distended periodically, would almost always be weighted with lumps 

 of sdid food. 



Within this ventral region we may suppose that, in addition to 

 the alimentary canal, we should have all the main trunks of the 

 circulatory system, the excretory organs (segmental nephridia) and 

 the genital bodies. Let us see how the new burdens which we 

 imagine to have been thrown on the alimentary canal might be 

 expected to affect not only these, but indirectly also the respiration, 

 which is always closely associated with the circulation. 



The Circulatory System. — In the paper on the Arachnida 

 above quoted I have already given an outline sketch of the profound 

 changes which the method of feeding of the Arachnida has necessi- 

 tated in the circulation of the different arachnidan families. No 

 less striking should be the changes produced in the circulation of 

 the primitive vertebrate when a fully distended stomach pressed 

 against the developing notochord dorsally, while ventrally and 

 laterally it stretched the skin to its fullest extent. We are justified 

 in assuming that the principal blood-vessels of our hypothetical 

 hirudinean ancestor ran longitudinally. I suggest that the dis- 

 tended alimentary canal would seriously hinder the passage of blood 

 along these vessels, and we might expect a congestion both in front 

 of and behind the obstructing swelling of the alimentary canal. The 

 anterior congestion of the vessels is that which alone concerns us. 

 It would lead to their distension both transversely and longitudin- 

 ally, the latter distension being accompanied by some degree of coil- 

 ing. It is further conceivable that at some portion of the congested 

 system a thickened muscular tunic would be developed to cope with 

 the difficulty, the thickening tunic perhaps involving the coils, so 

 that a specialised heart might be developed, capable of forcibly 

 pumping the blood past any such obstruction as the alimentary 

 canal could cause. That the anterior and not the posterior conges- 

 tion would give rise to the heart we may infer from its proximity to 

 the central nervous system, which would, doubtless, be in some way 

 connected with the muscles causing the pulsation of the primitive 

 vessels. 



Respiration. — The intimate connection which exists between 

 the circulation and the respiration is so well established that it needs 

 no emphasis here. I call attention to it merely to show that, if the 

 mechanism of the circulation became localised, as suggested, at the 

 anterior end of the body, so the respiration would tend to be local- 

 ised near the heart. Here again the diffused cutaneous respiration 

 of the Hirudinea, with their capillaries even entering the epidermis, 



