42 SCIENCE PROGRESS. 



As soon, however, as the first segment bent round ven- 

 trally, so that the mouth opened backwards, the pro- 

 stomium would take up the position of the labrum in 

 Apus and the Trilobites, and the ventral border would form 

 a small ridge-like under lip, also pointing backwards, such 

 as is actually found in Triarthrus. This under lip in 

 Triarthrus is thus another slight confirmation of our deduc- 

 tion of the primitive Crustacea from an Annelid with its 

 first segment bent round ventrally. 



This under lip of Triarthrus is however no longer the 

 simple ridge we must assume it to have been in its most 

 primitive condition. It already shows a tendency to form 

 lateral lobes. These small lobes, so insignificant in them- 

 selves, become important when studied comparatively. 

 Their remarkable development in Apus reveals to us their 

 function. They supply us with an interesting illustration 

 of the dependence of morphology upon physiological need. 

 In order to make this clear we shall have to make a brief 

 digression to explain the new method of feeding which 

 the bending round of the mouth implies. 



The chaetopod Annelids, with their anteriorly placed 

 mouths, largely feed by protruding an evaginable, and 

 generally armed, portion of the pharynx. This is thrust 

 out, seizes the prey, and is then withdrawn back again into 

 the mouth. Any new position adopted by the mouth 

 could only be in adaptation to some new method of feeding. 

 We have, therefore, to see what advantages would be 

 afforded by the bending round of the mouth ventrally 

 backwards. There is an immense amount of nutrient 

 matter, animal and vegetable, which, coating the rocks and 

 weeds, is quite unattainable by the shooting out of a 

 proboscis. Any Chaetopod, therefore, learning to browse 

 in such a way as to be able to take advantage of this hitherto 

 unattainable supply of food, would stand a good chance in 

 the race for life. I have, for instance, long thought that 

 the great phylum of the Mollusca might be deduced, in a 

 manner which I think can be worked out in detail, from 

 Chaetopods which early learnt to feed by scraping the sur- 

 faces of things with the ventral edge of the protrusible 



