240 SUMMARY OF CURRENT RESEARCHES RELATING TO 



described, but, unfortunately, only the diagnoses are given in English, 

 the full details being described in Norwegian. There is, however, 

 a brief description of the plates in English. 



Origin of Coral Reefs.* — Prof. A. Geikie sums up a considerable 

 amount of evidence which has accumulated since Charles Darwin's 

 theory on this subject was put forth, tending to show that the theory 

 (essentially that of growth of coral in connection with subsidence of the 

 sea bottom) is by no means universally applicable. Semper and Eein 

 supposed that in some cases raised masses of sand or deep-water corals 

 are formed which afford resting places for surface-growing corals ; the 

 form of the islands, Semper held, is caused by the death of the 

 inner parts of the colonies of corals, and by the action of the tides. 

 Mr. J. Murray, from observations made on the ' Challenger,' considers 

 that volcanic cones, such as form most oceanic islands, tend to be 

 reduced to submerged banks by the action of the waves ; also that 

 the raising of the sea bottom to such a height as to favour the 

 growth of corals, is due to the unusually rapid accumulation near the 

 shore of calcareous debris derived from dead pelagic organisms. 

 These are so abundant as probably to represent upwards of 16 tons 

 of carbonate of lime in suspension in the uppermost 100 fathoms of 

 every square mile of the ocean. In the deepest water these appear 

 to be dissolved before reaching the bottom, but they accumulate on 

 shallow bottoms, and thus furnish foothold for sponges, various 

 Ccelenterates, &c., which in return die and bring up the bottom to the 

 level of reef coral growth. This, taking place on a submerged bank, 

 would produce the atoll form of island, which would tend to widen 

 by death inside, and by the consequent solution of the dead coral by 

 the carbonic acid of the sea-water. Special cases, such as elongate 

 chains of atolls, e. g. the Maldives, or submerged banks, as the Chagos, 

 fall in with the theory. Barrier reefs are similarly explained as due 

 primarily to growth upon accumulations of debris around land. 



Porpitidae and Velellidae.t — We have here a notice of the work 

 of A. Agassiz on these little known Hydrozoa. Velella mutica, of 

 the coast of Florida, is much larger than the Mediterranean V. spirans, 

 and not unfrequently reaches 4 in. in length. It is exceedingly 

 common in Key West Harbour, which it visits in large schools. Feed- 

 ing is chiefly effected by the large central polypite of the system, and 

 this, together with the smaller polypites, is connected at its base with 

 the general vascular system, through which, as in the polypites, the fluid 

 is rapidly propelled by the lining cilia. At the base of the polypite 

 are the medusoid buds, and these, it is interesting to note, early 

 become provided with the yellow cells which are characteristic of the 

 free Medusae. The young present a striking resemblance to certain 

 Tubularian Medusae, being provided with a row of lasso-cells which 

 extend from the base of the tentacles to the abactinal pole. 



The Floridan species of Porpita (P. linneana) is, similarly, larger 

 than the Mediterranean P. mediterranea ; unlike Velella it has a 



* Nature, xxix. (1883) pp. 107-10. 



t Mem. Mus. Coinp, ZooL, 1883, See Nature, xxix. (1884) pp. 262-3. 



