THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



vol.. XVIII. SEPTEMBER, 1896. No. 9 



The Development of the Free-Swimming Medusae of Obelia 

 Commissuraiis. 



By GEORGE W. NORTON, 

 middi,h;town, conn. 



The development of the bell-shaped medusae has been 

 quite completely worked out, while that of the saucer- 

 shaped medusae, such as is found among the Campanu- 

 larian hydroids, has been studied but comparatively 

 little. The development, however, of theCampanularian 

 jelly fish, forms a no less interesting and instructive line 

 of study than that of their bell-shaped relatives, and 

 especially is this true if w^e make a comparative study 

 of the development of the two and note wherein they 

 agree and differ in their mode of development. 



EXPLANATION OF THE PLATES 



Fig. 1. A branch of a hydromedusarium. mal layers which break through and form the 



(a) the reproductive calycle. opening to the sub-umbrella cavity, (e) the 

 Fig. 2. The repioductive calycle highly sub-umbrella cavity. 



uiaguified. (a) the medusa, (b) the calycle. Fig. 9. The medusa ready to break loose 



(c) a young bud. from the manubrium of the calycle. (a) the 



Fig. 3. A section through a medusa bed in mouth, (b) the tentacle, (c) the circular 



an early stage, (a) the ectoderm, (b) the canal, (d) the stomach, 



endoderm. (c) thickening of the ectoderm . Fig. 10. A cross section of the bud in fig. 



Fig. 4. A section through a bud more ad- 8 as indicated by a. (a) the radial canals, 



vanced. (a, b, c) the same as in fig 3. (b, c, d) ectodermal layers, (e) the mouth or 



Fig. 5. A later stage of the bud shown in oesophagus, (f) the sub-umbrella cavity, 



fig. 4. (a) the cells forming from the ectu- (g, h, i) the eududermal layers, 



dorm, (b) the same as in fig. 3. Fig. 11. A fresh tentacle highly magnified 



Fig. 6. A later stage of the bud shown in (a) the tliread cells, 



fig. 5. (a) ectodermal cells arranged In two Fig. 12. A medusa at time of birth. The 



layers, (b) the same as in fig. 3, tentacles ar« here represented to be much 



Fig. 7. A more advanced stage of the bud shorter than they shonld be to be in propor- 



shown in fig. G. (a) the sub-umbrella cavity. tion with the rest of the body, (a) a tentacle. 



(b) the proboscis, (c) the stomach, (d, e) (b) a radial canal, (c) the circular canal, (d) 

 the endoderm. the mouth, (e) the proboscis, (f) an octocyat. 



Fig. 8, A further development of the same (g) the sub-umbrella cavity, 

 bud. (b) the proboscis, (c, d) the ectoder- 



