CHAPTER XV. 



PHORONIDEA. 



Although investigators early drew attention to the many structural 

 features in which the genus Plioronis resembles the Bryozoa, this form 

 has nevertheless hitherto usually been classed with the Sipunculids. 

 Recently, however, greater stress has been laid on its relationship 

 to the Bryozoa and the Brachiopoda (Ray Lankester, Caldwell 

 (No. 1), Cori (No. 4a)). It is chiefly in the anatomy of the adult 

 that the resemblance between these groups is found, but the larval 

 forms may also without difficulty be compared with one another. 



I. Embryonic Development. 



Our knowledge of the first ontogenetic stages of Phoronis is due 

 to the researches of Kowalevsky (No. 6), Metschnikoff (No. 9), 

 Foettinger (No. 5), Roule (No. 9a), and Caldwell (No. 2). The 

 latter author, whose description we shall follow in all important 

 points, arrived at results which frequently differ from those of earlier 

 investigators, so that many points seem to require re-examination. 



The eggs of Phoronis, according to Kowalevsky, are fertilised 

 while still in the body-cavity of the parent.* They reach the 

 exterior through the nephridial canals which open near the anus 

 and function as genital ducts, and then, enveloped in a vitelline 

 membrane, become attached to the tentacles of the parent, where 

 the young develop up to the time of hatching. 



Cleavage is total and unequal ; the difference in size between the 

 blastomeres of the animal and those of the vegetative pole is, how- 

 ever, inconsiderable. As early as the four-celled stage, two smaller 

 blastomeres can be distinguished from two larger; the eight-celled 

 stage shows four smaller and four larger cleavage-spheres symmetri- 

 cally arranged In the further course of this very regular cleavage, 



* Cori considers this statement improbable, and thinks that fertilisation 

 takes place outside the parent, in the water. 



