510 DB MASTERMAN ON THE FURTHER ANATOMY AND 



grooves of these which join in pairs to form the three oral grooves on each side. The 

 current of water and food-particles then passes through the peri-pharyngeal ring, into 

 the main body of the pharynx. It is difficult to conjecture whether the mucoid stream 

 passes forwards ventrally up this ring, and backwards dorsally, or whether the mucus, 

 emerging from the sub-neural gland, passes backwards both dorsally and ventrally, 

 but in either case, the food-particles become entangled in the mucus and are eventually 

 carried backwards through the narrow oesophageal opening into the stomach whilst the 

 water, flowing up to this opening, in the centre of the pharynx, will return as a back- 

 flow along the notochordal grooves. 



In Phoronis, as has been indicated in a preliminary note (7), the water-current is 

 got rid of by means of the epistome, but in Cephalodiscus a large quantity of water 

 must be carried into the pharynx, and the pharyngeal slits are admirably situated for 

 its removal. Their chordoid walls must serve to keep the clefts permanently open. 

 In addition to this, it appears probable that the dorso-lateral grooves which I have 

 termed pleurochords will also serve to carry off the superfluous water outwards and 

 forwards. The pharynx contracts to a small oesophageal opening at the hind end, and 

 the whole structure suggests that, whilst the mucoid strands, laden with food, pass on 

 to the stomach, a back current of water passes forwards along the cavities of the 

 pleurochords and out by the pharyngeal clefts (fig. 100). If this be so, then the 

 chordoid structure, both of the pharyngeal clefts and of the pleurochords may be due 

 to the necessity for rigid channels to allow of the escape of cloacal water. Whether as 

 paired pleurochords or as a median notochord, this function would persist in the 

 early Chordata till pharyngeal clefts were formed at the posterior end of the pharynx, or 

 as in Phoronis an extra-stomial means of separating the water from food were evolved. 

 The notochord would then persist only in those forms in which it had secondarily 

 acquired a supporting function (Euchorda). 



Thus the loss of pleurochords in Phoronis may be traced to the peculiar adaptation 

 of the epistome and the preclusion of a saving secondary function owing to the sedentary 

 habitat, whilst the loss of the notochord in the Urochorda is primarily due to the 

 removal of the necessity for the secondary supporting function, involved in a like 

 manner in the adoption of a sedentary habit. 



Thus one is led to suggest that the notochord of the Chordata primarily owed its 

 origin to the necessity for a rigid channel in the endodermal walls of the pharynx, 

 intimately connected with the 'primitive process of ciliary ingestion of food. The 

 function of the notochord as an elastic primary axis to the body would thus be << 

 secondary modification. 



This assumption appears to be justified by the fact that the pleurochord of Cephalo- 

 discus, permanently in the condition found in the embryonic Euchorda alone, i.e 

 an endodermic longitudinal groove, most probably fulfils this function. 



All the Chordata except the Vertebrata (or Holochorda) carry on their ingestive 

 processes by ciliary action, with the consequent necessity for a separation of the water 



