412 ARTHUR T. MASTERMAN. 



The first of these is that Actinotrocha has a pair of 

 organs which have at least as great a claim to be regarded 

 as rudimentary chordoid oi'gans allied to the notocliord of 

 the Vertebrates as have the so-called notochord of Balano- 

 glossus and Cephalodiscus. Roule (15a) has confirmed 

 the chordoid structure of them, although he differs in other 

 points. 



In 1897 (10) it was pointed out that the chordoid structure 

 of the gut in Actinotrocha was not confined to the two 

 diverticula, but was also found in the mid- ventral portion of 

 the stomach just opposite the sac-like invagination which 

 later gives rise to the trunk of the adult. ^ This alone showed 

 the need for caution in drawing too close homologies, and 

 although the hypothesis was made that the two pleurochords 

 represented the paired rudiments of the single notochord of 

 the Vertebrates, it was with some reservation. 



A study of Balanoglossus itself shows that the chordoid 

 metamorphosis of the gut epithelium is very widely present 

 in the various parts of the alimentary canal, especially in the 

 pharynx, in parts of which it extends completely round the 

 whole pharyngeal wall. This is so easily demonstrated (it is 

 referred to by Spengel) that it is surprising-, firstly, that 

 the fact is universally ignored in text-books ; and secondly, 

 that in the face of it the small pre-oral diverticulum alone 

 should have been selected for comparison with the notochord 

 of the Vertebrata (cf. Bateson). Consideriug that the 

 organ lies pre-orally, its great claim for notochordal recog- 

 nition must rest in the fact that its cells are metamorphosed 

 into vacuolated tissue similar to that of the vertebrate 

 notochord, and the value of this comparison is considerably 

 lessened when it is considered that the dorsal part of the gut 

 in the collar region, and even beyond it, is chordoid, and is 

 in a position relative to the nervous system more closely 

 resembling that of the vertebrate notochord. The structure 



1 This appears to be in the same position as the so-called " pygochord " of 

 Willey, occurring in certain species of Enteropneusta. — A. T. M., March, 

 1900. 



