10 PHORONIDEA. 



sand or even in a tube, a sliifting similar to that wliicli takes place 

 in the Siinmrulidae {YoX. i., p. 363). 



The fact that, in the trunk region of the adult Phoronis the 

 symmetrical arrangement of the body begins to be disturbed, is no 

 doubt connected Avith the tubicolous manner of life. There is thus- 

 a longitudinal nerve-strand on the left side, and, according to CoRi 

 (j^o. 4), a new plane of symmetry, different from the original one, 

 is evident in transverse sections in the grouping of the longitudinal 

 muscles in Plioronis psammojjhila. Indeed, in the presence of 

 secondary and tertiary mesenteries, which leave between them distinct 

 intervals containing the groups of longitudinal muscles, there is a 

 tendeircy to the development of the radial t3'pe. 



It should here be pointed out that, in Plioronis, the cephalic region 

 is often lost and again regenerated. According to CoRi (No. 4a), 

 not on]}' the tentacle crown, but Avith it a part of the oesophagus^ 

 the epistonie, the ganglion, the so-called lophophoral organs, the 

 blood-vascular ring, and perhaps also the nephridia are lost. The 

 spontaneous throwing off of the cephalic section is specially liable 

 to occur Avhen the animals are placed in unfavouralile conditions- 

 of existence. This process closely resembles the throwing off of 

 tlie head in Pedicellina and in the Tubularia ; and the same process- 

 is found in the Ectoproctous Bryozoa, in the disintegration and 

 subsequent regeneration of the polypide (p. 55). 



LITERATURE. 



1. Caldwell, W. H. Preliminary note on the structure, develop- 



ment and affinities of Phoronis. Proc. R. Soc. London. 

 Vol. xxxiv. 1882-1883. 



2. Caldwell, W. H. Blastopore, Mesoderm, and Metameric Seg- 



mentation. Quart. Journ. Micro. Set. 1885. Vol. xxv. 



3. Claparede, E. Beobachtungen liber Anatomie und Entwick- 



lungsgeschichte der wirljellosen Thiere. Leip':ig. 1863. 



4. CoRi, C. J. Beitrag zur Anatomie der Phoronis. Inaug.-Diss. 



Pray. 1889. 

 4a. CoRi, C. J. Untersuchungen liber die Anatomie und Histologie 

 der Gattung Phoronis. Zeitschr. f.Wiss. Zool. Bd. li. 1891. 



5. FoETTiNGER, A. Note sur la formation du mesoderme dans la 



larve de Phoronis hippocrepia. Archiv. de Biol. Tom. iii. 1882. 



6. KowALEVSKY, A. On the anatomy and development of Phoronis 



(Inaug.-Diss. Kussian). Petersburg, 1897. See Leuckart's. 

 Summary in Jahresh. Archiv. Naturg. Vol. xxxiii. 1867. 



