250 SIDNEY F. HARMER. 



tion of a Polyzoon larva by any other than its oral surface, 

 and it may therefore be assumed that this method of fixation 

 was acquired at a very early stage in the phylogeny of the 

 group. Let us suppose, however, that this "Archi-Loxo- 

 soma," on fixing itself by the edge of its vestibule, left an 

 aperture (for the entrance of food), surrounded by the ciliated 

 ring {vide fig. 17), leading from the exterior into the otherwise 

 closed vestibule, and situated behind the anus. 



Subsequent development may be imagined to give rise to a 

 form like fig. 18, in which the vestibular opening is an elon- 

 gated slit, extending along the whole of the region formerly 

 occupied by the posterior side, and still surrounded by the 

 ciliated ring. The mouth, in order to obtain its food as con- 

 veniently as possible, now faces the posterior side (of the 

 former stage), and this has entailed a rotation of the entire 

 alimentary canal, in the manner shown in fig. 18. 



By the growth of the proximal end of the Polyzoon, the 

 raouth would be thrust away from the point of support, and 

 the animal might thereby obtain an advantage in procuring 

 food by means of its ciliary currents. But during this process, 

 the proximal portions of the ciliated ring would become far 

 less efficient for obtaining food than the distal portions, and 

 would tend to atrophy. The final result would be the acquire- 

 ment of a form like fig. 19, representing in a very slightly 

 diagrammatic form, an adult Loxosoma. The ciliated ring is 

 here represented as consisting of two disconnected portions, cor- 

 responding (1) to the ring of tentacles ; (2) to the foot-gland (cf. 

 the second of Barrois* conclusions quoted on p. 248) . The foot- 

 gland has remained practically as an open groove, a series of 

 ciliated tentacles having been developed round the margin of 

 the permanent vestibule. 



The position of the buds in the larval Loxosoma appears at 

 first sight fatal to the above hypothesis. That this larva does 

 actually develop buds normally can hardly be doubted, since 

 I have shown not only that these structures are developed 

 twenty-four hours after hatching (which might, however, be an 

 abnormal circumstance, due to the want of proper conditions 



