ON THE DIPLOOnORDA. 357 



subneuval gland and the notochordal diverticula (fig. IG), and 

 that a breaking-down of the terminal cells gave rise to the 

 clefts. Vacuolisation is certainly a process of cell-degene- 

 ration, and disintegration of the vacuoles would be but a step 

 further in the process. On this hy])othesis the notochordal 

 diverticula of Actinotrocha might possibly be interpreted as 

 a pair of blind pharyngeal clefts in process of formation. 



The objection to this theory is that it implies a discontinuity 

 in function, the pharyngeal clefts being incapable of performing 

 their true function until they were actually formed, an objection 

 which applies equally heavily to the hypothetical formation of 

 a new mouth and a new anus in the Vertebrata, and the 

 annelid derivation of this group. 



The other alternative is to suppose that the pharyngeal clefts 

 were already present without chordoid support, and that the 

 lining cells became vacuolated later. 



It is in these lowest Chordata that one would expect the 

 chordoid tissue to be diffuse and occurring in any part of the 

 hypoblast where it is specially needed (cf. Actinotrocha). 

 This explanation of the chordoid pharyngeal clefts appears to 

 me to be safe, and to leave out of the question the primary 

 origin of gill-slits. 



It is remarkable that there are no gill-slits in Phoronis, 

 though reasons have been given elsewhere for their absence in 

 the adult (see below). In Actinotrocha there are a pair of 

 atrial grooves (see Actinotrocha) which function for the re- 

 moval of water brought along the oral grooves by ciliary action. 

 These grooves, therefore, have the function which the pharyn- 

 geal clefts perform in Cephalodiscus, and it is conceivable 

 that these grooves travelled back on either side in the course 

 of phyletic history of the Chordata, performing their function 

 the more efficiently as their point of exit from the pharynx 

 became carried further backwards. The tissue behind their 

 track would close up, and no trace of the migration of the grooves 

 (now clefts) would remain. There seems to me to be no more 

 difficulty in this process than in the migration of the origin of 

 a blood-vessel from one point to another of the parent vessel. 



