6 



PHORONIDEA. 



the first to prove that the Adinotrorlia was the young form of 

 the PJioivnis discovered by Wright. Since that time, the meta- 

 morphosis of Phoronis has been more accurately investigated by 

 Metschnikoff (No. 8), Wilson (No. 11), and Caldwell (No. 1). 

 The first changes that take place in the Adinotrocha consist of 



a simple increase in size tlirough growth 

 and a continual increase in number of 

 the tentacles. At the same time, a 

 sensory organ develops in front of the 

 neural plate, four eye-spots being added 

 to it in one species. The pigment-spots 

 characteristic of the different species 

 now also develop on the pre-oral lobe 

 and on the tentacles. 



Rudiments now appear of the de- 

 finitive structures that are destined to 

 replace the larval organs. Of these, 

 the first to develop is an invagination 

 of the body-Avall on the ventral surface 

 of the posterior region of the larval 

 body (Fig. 4 C, iv), and in this tlie two 

 layers of the body-wall (the ectoderm 

 and the somatic mesoderm) can be distinguished. This invagination, 

 Avhich soon grows as a much-coiled tube into the larval body-cavity, 

 represents, as Ave shall see, the rudiment of the greater part of the 

 body-wall of the adult. Small truncated processes now develop at 

 the base of the crown of tentacles, and from these are derived 

 the adult tentacles (Fig. 4 D). 



When these structures have appeared as rudiments, the Adino- 

 trocha sinks to the bottom, the critical moment of the commencement 

 of metamorpliosis having arrived, the whole process being accom- 

 plished Avithin a quarter of an hour. ISIetamorphosis is introduced 

 by the evagination of the tube mentioned above (Fig. 4 D), this 

 being ju-otruded like the tentacle of a snail. Since the alimentary 

 canal, together AA'ith its mesenteiy, is attached to the inner end of 

 this tul)e, it soon has to folloAV the movement thus begun, and so 

 comes to lie inside the completely evaginated tube (Fig. 4 E). 

 During these changes the rest of the larval body loses its turges- 

 cence. The oral and anal apertures therefore come to lie remarkably 

 near one another. The pre-oral lobe of tlie larva is noAv throAvn off, 

 and the same fate overtakes the larval tentacles and the circum-anal 



ari' 



FiQ. 3. — Larva of I'horonis (Aclino- 

 trocha), after Metschnikoff, 

 from Balfour's Text-hook, m, 

 mouth ; an, anus. 



