358 A. T. MASTERMAN. 



In ontogeny one would hardly expect to find the exact process 

 repeated, for other structures may have been evolved in the 

 areas lying in the course of the migration, which appear early 

 in the ontogeny, but there are certain facts in the development 

 of gill-slits which may be explained in the light of this hypo- 

 thesis. Thus the gill-pouches of Tornaria first appear as 

 paired sac-like outgrowths of the oesophagus close to the mouth 

 region, so much so that if they reached to the overlying epi- 

 blast immediately on their appearance and perforated it, the 

 gill-slits would open through the lateral wall of the proboscis 

 (cf. Morgan, ' Journal of Morphology,' January, 1894). Later 

 on they shift along with the gut itself posteriorly, so that by 

 the time they reach the epiblast they do so at a spot behind the 

 collar-pores, and in the anterior part of the trunk region. This 

 is the kind of modified repetition we should expect, in ontogeny, 

 of a phyletic migration of apertures from one part to another 

 (cf. formation of new mouth in An ted on and other types). 



At any rate tentatively, I would regard the atrial grooves of 

 Actinotrocha as the early rudiments of pharyngeal clefts as 

 found in Cephalodiscus. 



The structural condition of the endoderm in the pharyngeal 

 region of Actinotrocha, Cephalodiscus, and Balano- 

 glossus may be compared as follows: — In PI. 26, fig. 2i, is 

 seen a diagrammatic transverse section of the pharynx of 

 Actinotrocha. In this case diff'erentiation of the endoderm 

 has taken place. Two lateral areas are modified into chordoid 

 tissue for supporting function, whilst the ventral ciliated area 

 is more directly connected with alimentation. The two crosses 

 indicate the position homologous to that in which pharyngeal 

 clefts are situated in the other two types. 



In Cephalodiscus (fig. 22) the same two lateral chordoid 

 areas are seen, though more approximated in the mid-dorsal 

 line. In addition, the two chordoid pharyngeal clefts open at 

 their ventral border, and below these, in the mid-ventral line, 

 is the special alimentary part or true gut.^ 



* The right-hand side is supposed to indicate the condition behind the gill- 

 slits (cf. Actinotrocha, fig. 21). 



