PHOROXIDEA. 



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

 the Phoronis discovered by AVright. Since that time, the meta- 

 morphosis of Phoronis has been more accurately investigated by 

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

 The first changes that take place in the Actinotrocha consist of 

 a simple increase in size through 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. 



Eudiments 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-wall on the ventral surface 

 of the posterior region of the larval 

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

 layers of the body-wall (the ectoderm 

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

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

 represents, as we 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 Actino- 

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

 of metamorphosis having arrived, the whole process being accom- 

 plished within a quarter of an hour. Metamorphosis is introduced 

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

 being protruded like the tentacle of a snail. Since the alimentary 

 canal, together with its mesentery, is attached to the inner end of 

 this tube, it soon has to follow the movement thus begun, and so 

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

 Inuring these changes the rest of the larval body loses its tumes- 

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

 near one another. The pre-oral lobe of the larva is now thrown off, 

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



CC7V 



Fig. 3. — Larva of Phoronis (Actino- 

 trocha), after Metschnikoff, 

 from Balfour's Text-book, m, 

 mouth ; an, anus. 



