142 Papers from the Department of Marine Biology. 



In Asterina (Bury, 1), while the anterior enterocoele is essentially a 

 double structure, there are three communications between them a 

 ventral just below the hydroccele and two dorsal, one just above the 

 hydrocoele and a second behind the pore. In Laganum, although the 

 body of the embryo is compressed dorso-ventrally and the lumen of 

 the cavity in that region is consequently very narrow, there seems 

 to be no separation of the right and left portions into distinct lobes. 



BILATERAL SYMMETRY. 



It is evident that in Laganum there exists a bilateral symmetry not 

 characteristic of other forms of echinoderm larvae at a corresponding 

 stage of development. The cavities of the anterior enteroccele at 

 each side of the gut are usually of about equal size and extend approxi- 

 mately the same distance towards the posterior end. The ring of the 

 hydroccele lies immediately in front of the centrally located gut, and 

 its lobes have a perfect bilateral symmetry with regard to the median 

 plane of the animal. On each side of the posterior primary tentacle lies 

 one of the vesicles which together represent the posterior enteroccele. 

 The only interruption of the bilateral symmetry is caused by the pore 

 canal which, arising from the left side of the hydroccele ring and circling 

 around the gut, opens to the exterior, well to the left side of the pluteus. 



Whether this symmetry, which has not been described for so late a 

 stage of the echinoderm, is a primary or a secondary development 

 could of course be definitely determined only by a study of earlier 

 stages. But, however this condition arises, it is evident that we have 

 here a form unusual not only in regard to this characteristic, but also 

 in respect to other fundamental properties, such as the rapidity of early 

 growth, the great complexity of the larval skeleton, the formation of 

 the amniotic cavity from a central invagination, the entire absence of 

 right posterior enteroccele and hydroccele, and the complete constriction 

 of the left posterior enteroccele into two separate vesicles. 



BRYN MAWR COLLEGE, January 8, 1915. 



BIBLIOGRAPHY. 



1. BURY, H. Studies in the embryology of echinoderms. Quart. Journ. Micr. Sci., vol. 29. 1889. 



2. GOTO, S. The metamorphosis of echinoderms, with special reference to the fate of the body 



cavities. Journ. Coll. Soc. Imp. Univ., Tokyo, vol. x. 



3. MACBHIDE, E. W. The development of Echinus esculenlus. Phil. Trans. Roy. Soc., vol. 



cxcv. 1903. 



4. MACBRIDE, E. W. The development of Echinocardium cordatum. Quart. Journ. Micr. Sci., 



vol. 59, N. S., No. 236. 1914. 



5. PEACE, L. M. Clearing and staining leaves and stems. Plant World, vol. 13, No. 4. Apr., 1910. 



6. TENNENT, D. H. Echinoderm hybridization. Carnegie Inst. Wash. Pub. No. 132. Jan. 1911. 



7. THEEL, H. On the development of Echinocyamus pusillus. Nova Acta. R. Soc. Sci., 



Upsala. 1892. 



