10 PHORONIDEA. 



sand or even in a tube, a shifting similar to that which takes place 

 in the Sipunculidae (Yol. 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 with the tubicolous manner of life. There is thus 

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

 (JSTo. 4), a new plane of symmetry, different from the original one, 

 is evident in transverse sections in the grouping of the longitudinal 

 muscles in Phoronis psammophila. Indeed, in the presence of 

 secondary and tertiary mesenteries, which leave between them distinct 

 intervals containing the groups of longitudinal muscles, there is a 

 tendency to the development of the radial type. 



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

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

 not only the tentacle crown, but with it a part of the oesophagus, 

 the epistome, 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 when the animals are placed in unfavourable conditions 

 of existence. This process closely resembles the throwing off of 

 the 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. Sci. 1885. Vol. xxv. 



3. CLAPAREDE, E. Beobachtungen iiber Anatomic und Entwick- 



lungsgeschichte der wirbellosen Thiere. Leipzig. 1863. 



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



Prag. 1889. 



4a. CORI, C. J. Untersuchungen iiber 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. Arcliiv. de Biol. Tom. iii. 1882. 



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



(Inaug.-Diss. Russian). Petersburg, 1897. See LEUCKART'S 

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



