360 A. T. MASTBRMAN. 



into tnie gill-slits (branchial) with mc^soblastic skeletal bars, 

 the chordoid tissue evolved into a supporting organ for body- 

 muscles, and eventually itself replaced by chondroid vertebral 

 tissue, and the ventral groove, the true alimentary part (gut) 

 in the Archichorda, losing its connection with the gut, and 

 becoming more or less vestigial as the thyroid gland. We 

 can clearly see some of the lines in relation to function along 

 which these changes have proceeded. In the Archichorda 

 the method of food ingestion is, typically, by ciliary action, 

 causing a current of water and minute food particles. The 

 former has to be removed by pharyngeal clefts (Harmer, Brooks), 

 or by atrial grooves, in Actinotrocha, whilst the latter is 

 entangled in currents of slime, and thus retained and passed 

 down the gut for digestion. In Phoronis the same method 

 of ingestion is effected, but the water-current is got rid of by 

 a special adaptation of the epistome, to be described later, 

 before the mixed currents reach the true mouth, so that true 

 pharyngeal clefts should be superfluous ; and not only so, but 

 as the pharynx is an organ especially evolved for the effectual 

 separation of these two currents, it has entirely disappeared 

 in Phoronis, carrying with it its chordoid walls. 



The same method of ingestion is pursued in the Uro- 

 chorda and even the Cephalochorda, though in each of 

 these, as in the Hemichorda, the simple pharyngeal clefts are 

 elaborated into a complicated system of branchial slits. 



In the Holochorda the mesoblastic gill-bars are impressed 

 into the service of food ingestion, and the Gn athostomata 

 are evolved. The true jaws and elaborated locomotory system 

 enable more bulky prey to be secured, and the primitive func- 

 tion of pharyngeal clefts, that of removal of the water-current, 

 is no longer existent, so that the more recently acquired 

 branchial function alone warrants their maintenance. A few 

 exceptions are noteworthy. In the herrings and their allies, 

 the diet ofCopepoda has given rise to a return of this func- 

 tion, and the gill-slits and gill-rakers are again requisitioned 

 for removal of water-current. Again, in the Cetacea, a 

 return to the pelagic diet involves the elaboration of a systern 



