1898] ANNULATE ANCESTRY OF THE VEBTEBRATA 21 



to ensure the purity of the food have developed a beautiful variety 

 (if sieves and strainers to prevent any solid matter from passing'into 

 the alimentary canal. With this purely liquid food, they manage 

 to distend themselves almost to bursting, by a kind of force-pump 

 action of the oesophagus. The effect of this distension of the 

 alimentary canal upon itself and upon the other organs of the body 

 can be made to explain all the more important structural pecu- 

 liarities and variations in the Arachnida. No single organ or 

 assemblage of organs has remained unaffected ; all have had to adapt 

 themselves or protect themselves. The whole form of the body has 

 been changed, limbs have aborted, and the respiratory and circulatory 

 systems have been profoundly modified. So obvious is this to any 

 une who studies the group from this point of view, that we are quite 

 justified in postulating modifications and adaptations of the organisa- 

 tion of our assumed hirudinean ancestor, not only to the full-meal 

 condition of its alimentary canal, but also to its more constant load 

 of solid lumps. 



In the Arachnida, one necessary precaution was the protection 

 of the muscular and nervous apparatus against temporary incapacity 

 due to the distension of the alimentary system. This has been 

 brought about by a division of labour, one region of the body under- 

 taking the animal (locomotory and sensory) functions, the other the 

 vegetative and digestive functions. The arachnidan body accordingly 

 is divided transversely by a narrow waist or diaphragm; the alimen- 

 tary canal in the posterior division can be distended to its utmost 

 limit without pressing at all on the anterior part. The cpaestion as 

 to which region should be the animal and which the vegetative was 

 naturally settled by the fact that the muscular apparatus of the 

 jaws and of the capturing limbs, and the ganglia of the great sen- 

 sory organs were already at the anterior end of the body. 



Xow I assume — and in this assumption lies my new reading of 

 the facts — that our supposed hirudinean had to undergo modifications 

 for precisely the same end, viz., the protection of the locomotor 

 functions from the alimentary. I again suggest that a division of 

 labour took place, the body dividing not transversely, as in the 

 Arachnida, but longitudinally, i.e., into a dorsal and a ventral half; 

 and then, that the dorsal half had to protect itself from the 

 ventral. 



We will deal first of all with this assumed division of labour. 

 The weight of the distended abdomen pressing downwards upon the 

 primitive ventral nerve-cord and muscles would seriously affect 

 their working ; while, as some compensation for this loss of power, 

 the dorsal neuro-muscular system, on which at any rate the 

 pressure due to gravitation did not act, would be free to fulfil its 

 functions, and thus able to develop in order to meet the greater strains 



