PHORONIDEA. 547 



constitute a body-cavity for the larva, but also give rise to the body- 

 cavity of the adult Phoronis. 



After a certain period of free life the larva sinks to the bottom, 

 and undergoes a sudden change of form and character which 

 constitutes the metamorphosis. The ventral invagination becomes 

 evaginated, and forms a tubular projection standing out from the 

 ventral surface almost at right angles to the long axis of the larva 

 (Fig. 439, a). Into this projection, which will constitute the 

 body of the adult, and which soon becomes larger than the larval 

 body, the alimentary canal passes and forms the U-shaped loop 

 characteristic of the adult. At the same time the hind part of 

 the larval body, or, as we may now call it, the anal papilla, 

 becomes less and less conspicuous, and eventually reduced to the 

 anal projection of the adult. While these changes are going on, 

 the preoral lobe with its ganglion and sense-organs passes into the 

 stomach by the oesophagus and is digested. The larval tentacles 

 likewise pass into the stomach. Meanwhile the adult tentacles have 

 become fully formed, attachment effected by the aboral apex of the 

 new body and the adult form acquired. 



Phoronis is exclusively a marine animal, and about 6 species are 

 known. 



Phoronis S. Wright ; Ph. hippocrepia S. Wright, embedded in coral or lime- 

 stone, Devonshire Coast, Firth of Forth ; Ph. kowalevskii Caldwell, tubes coated 

 with sand, Mediterranean. 



It has been suggested by Caldwell and others that Phoronis has affinities to 

 the Polyzoa and to the Brachiopoda. According to this view in all these 

 animals the dorsal surface of the body is reduced to the space between the 

 mouth and the anus, and the preoral lobe of the larva almost or entirely 

 disappears in the adult. There is much to be said for this view, which is 

 fully discussed under the two groups concerned (pp. 571 and 584). 



A. T. Mastennan has quite recently (Q. J. M. S., 40, 1897, p. 281) endeavoured 

 to show that Actinotrocha has Enteropneust features. The principal data upon 

 which he bases his view are partly new and partly old. The old are the presence 

 of two glandular pockets opening into the anterior end of the stomach, which 

 he identifies as a double notochord ; on what substantial grounds it is difficult 

 to see. The new data are (1) the presence of three sections of the body-cavity: 

 (a) one, unpaired, situated in the preoral lobe and opening to the exterior by 

 two pores placed on each side of the preoral ganglion, (b) a second chamber 

 which he calls collar body-cavitydivided by a dorsal mesentery but not by a 

 ventral, and opening to the exterior ventrally by a pair of nephridial tubes ; 

 and (c) a posterior chamber divided by a ventral mesentery and possessing paired 

 nephridia or the rudiments of them. (2) The presence of an ectodermal pit in 

 front of the preoral ganglion, which he calls the neuropore ; and (3) the presence 

 of an ectodermal pit on the ventral side of the preoral lobe, the opening of 

 which shifts into the stoniodaeum, and which he calls the subneural gland. 



