1898) ANNULATE ANUESTRY OF THE VERTEBRATA 25 
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 solid 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. Itis further conceivable that at some portion of the congested 
system a thickened muscular tunic would be developed to cope with 
the difticulty, 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, 
