ON THE DIPLOCHORDA. 361 



of whalebone bars, wbicb, in an animal whose ancestors long 

 since lost their pharyngeal clefts, perform the primitive func- 

 tion of these organs. 



To this change of diet (made possible by the mesoblastic 

 branchial bars ^) we may trace the change of function of the 

 pharyngeal clefts, and upon a loss of this later function (bran- 

 chial) their extinction. On the other hand, the notochord 

 may be traced from chordoid tissue supporting the ingestive 

 pharynx to a fused chordoid rod forming the main skeletal axis 

 until it also is replaced by a mesoblastic skeletal organ. 



The mid- ventral alimentary area of the Archichordate 

 pharynx, adapted for the secretion of mucus and the passage 

 of it, together with food-particles, eventually down the gut 

 (cf. endostyles, hypopharyngeal groove), loses its function in the 

 Gnathostomata, and retaining some other function of which 

 little is known, and which is not connected directly with that 

 of alimentation, is no longer in continuity with the gut. 



As already mentioned, I propose to include Balanoglossus, 

 Cephalodiscus, and Phoronis together in one group, the 

 Archichorda, dividing this secondarily into two sub-groups: 



1. Diplochorda, — Cephalodiscus, and Phoronis. 



2. Heraichorda, — Balanoglossus. 



Such differences as may hold between these three types can 

 be traced to their differing environment. Their common 

 meeting-ground is in a pelagic ancestor, very nearly represented 

 by Actinotrocha. We may suppose that this ancestor on 

 taking to life on the bottom developed a sucker on the ventral 

 surface, by which it was capable of fixing itself, more or less 

 permanently, to foreign objects. In the case of Phoronis, 

 the sedentary habit became complete, with a consequent 

 loss of pre-oral lobe, notochord, and sense-organs, and an 

 approximation of mouth and anus at the end furthest away 

 from the foreign object. In this particular case, the fixing 

 organ being ventral, the approximation is dorsal. In the case 



' Other factors in the evolution of the Gnathostomata are the increase in 

 size, made possible by greater correlation of parts and elaboration of tlie 

 muscular system. 



