x PODAXONIA 385 



amputated ; and that this proceeding is equivalent to the casting off 

 or absorption of the prototroch in other forms. 



If this supposition be justified, then the rest of the metamorphosis 

 can be viewed as a modification of the process of gradual growth 

 of the ventral part of the body, already observed in Phascolosoma 

 and Sipunculus. It is a very instructive modification, showing the 

 kind of secondary change which may be expected to occur in ontogeny. 

 It is another example of the complete omission of the intermediate 

 stage of development between the Trochophore stage and the adult 

 condition. This intermediate stage must have existed in the history 

 of the race and doubtless occurred at one time in the history of the 

 individual. 



The general conclusion then, to which we are led by a review of 

 the development of Phoronis, is that it really does belong to the 

 group of Sipunculus and Phascolosoma, and that the classification of 

 the older authors is so far justified. This conclusion, however, raises 

 another series of most interesting questions. The structure of 

 Phoronis is so similar to that of the Phylactolaematous Polyzoa 

 that Laukester (1890) regarded Phoronis as a Polyzoan ; and it seems 

 difficult to evade this conclusion. But in that case Phoronis would 

 be the only solitary Polyzoan known, and all the true Polyzoa 

 (leaving out of account the anomalous Eutoprocta) must be regarded 

 as modified Podaxonia. 



LITERATURE CONSULTED 



Caldwell, W. H. Preliminary Note on the Structure, Development, and Affinities 

 of Phoronis. Proc. Roy. Soc. (Lond. ), vol. 34, 1883. 



Caldwell, W. H. Blastopore, Mesodenn, and Metameric Segmentation. Quart. Journ. 

 Mic. Sci., vol. 25, 1885. 



Gerould, J. H. Studies on the Embryology of the Sipunculoidea, I. The Embryonic 

 Envelope and its Homologue. Mark Anniversary Volume, 1903. 



Gerould, J. H. Studies on the Embryology of the Sipunculoidea, II. The 

 Development of Phoscolosoma. Zool. Jahrb. (Anat. u. Out.), vol. 23, 1907. 



Goodrich, E. On the Body - cavities and Nephriclia of the Actinotrocha Larva. 

 Quart. Journ. Mic. Sci., vol. 46, 1903. 



Hatschek. Uber die Entwieklung von Sipunculus nudus. Arb. Zool. lust. Wim, 

 vol. 5, 1883. 



Ikeda. Observations on the Development, Structure, and Metamorphosis of 

 Actinotrocha. Journ. Coll. Sei. Imp. Univ., Tokyo, vol. 13, 1901. 



Lankester, E. R. Zoological Articles "Polyzoa," repub. from Encycl. 

 Britt,, 1890. 



Masterman. On the Diplochorda, Parts I. and II.' Quart. Journ. Mic. Sci., vol. 

 40, 1897 ; Part III., ibid. vol. 43, 1900. 



Metschnikoff, El. Uber die Metamorphose einiger Seetiere, III. Actinotrocha. 

 Zeit. AViss. Zool., vol. 21, 1871. 



de Selys-Longcbamps. Recherches sur le developpement de Phoronis. Arch, dc 

 biol., vol. IS, 1902. 



Shearer-Cresswell. Studies on the Development of Larval Nephridia (I. Phoroni , . 

 Mitt. Zool. St. Neapel, vol. 17, 1906. 



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