20 NATURAL SCIENCE [July 



In what follows, therefore, I propose to apply the last-named 

 method to the Hirudinea, and to enquire whether the structural 

 changes which might be expected to follow from a further develop- 

 ment of their method of feeding would not transform them into Verte- 

 brata. I am quite aware that the argument itself may from first to last 

 be merely an academical discussion without direct value to Morpho- 

 logical science. But it has been suggested to me that merely as an 

 essay in Physiological adaptation, especially as it is here applied to 

 a ' burning ' question, it may do good, by calling attention to this 

 method itself, and perhaps lead to useful discussion as to the sound- 

 ness of the principle and the extent of its applicability. 



Let us then assume a race of hirudineans not yet so specialised 

 as our medicinal leech with its dorso-ventrally flattened body and 

 terminal sucker, probably a further specialisation of this flattening. 

 We will assume that they lived freely in the open sea, chasing their 

 prey with true serpentine motion, and attacking it with open mouths, 

 armed with buccal teeth. Let us assume that this method of 

 obtaining food was eminently successful, no very arbitrary assump- 

 tion, because we may, as a rule, assume that a new method of feed- 

 ing is adopted, because a rich and hitherto untapped food-supply 

 offers itself. Let us, then, consider how such animals as we have 

 pictured, viz., free-swimming rapacious leeches, might possibly de- 

 velop, given an abundant food- supply. 



The first results would be growth in size, larger mouths and more 

 teeth. 1 These would lead to a very important change in the 

 character of the diet — viz., to the swallowing of a great deal of solid 

 food. Small animals would be gulped down whole, while lumps 

 would be torn out of larger ones too big to swallow. This change 

 would profoundly influence the alimentary system. Solid 

 food demands a slow passage down the alimentary canal while it is 

 being digested, and a full meal of such solid foods would entail a 

 great distension of the anterior section of the alimentary canal, the 

 ultimately differentiated stomach. 



Now it seems to me that a swollen anterior portion of the 

 alimentary canal, heavy with solid food — and we have every reason 

 to believe that the full meal was a frequent occurrence — would 

 necessitate, besides the formation of a muscular stomach, other and 

 more profound changes in the organisation. In the paper above 

 referred to on the morphology of the Arachnida, I have endeavoured 

 to show that almost every detail of the organisation of that group of 

 animals has been profoundly modified by the necessity for adapta- 

 tion to the full-meal condition. Unlike our hypothetical hirudinean 

 ancestors, the Arachnida have never given up pure blood-sucking, and 



1 The change from chitin to horn might be correlated with the change from the iini- 

 laminate to the mnltilaminate epidermis. 



