APPENDIX. 



ADDENDA AND ANNOTATIONS. 



Reproduction in Hie Hydroidea. p. 22. 



THE observations of Van Beneden, on the Tubularida;, but lately published, have 

 brought out many new facts with regard to the structure and modes of reproduction cha- 

 racterizing this division of the Hydroidea.* His investigations have led him to distin- 

 guish the following modes of reproduction : 



1 . By persistent buds, by which, as in the Sertularine, and in zoophytes generally, 

 compound groups are formed. 



2. By caducous ovule-like buds or gemmules. These are produced about the bases of 

 the tentacles, and have been considered true ova (pp. 22, 23), to which they are closely 

 analogous. Van Beneden describes them as presenting within, when complete, a distinct 

 cellule, which he considers as corresponding to the germinant vesicle of the true egg. 

 This cellule enlarges, and shortly a membrane forms across, which is in contact below 

 with the circulating fluids of the axis; from this membrane the new polyp proceeds. He 

 traces out the changes in progress from this state to the developement of the medusa-like 

 young, a Beroe in form, a floating pellucid disk, fringed around with delicate tentacles, 

 and furnished with eight eyes. 



3. By a single ovule, thus approaching in character the Actinoidea. 

 Besides the above modes, he mentions also two others. 



4. Compound ova, resulting in each instance from a production of numerous ovules 

 from the yolk of what at first appears to be a simple ovum, each ovule having its own 

 germinal vesicle, and producing separate young. 



5. Ovules formed within the caducous gemmules. This mode corresponds nearly to 

 known instances of ova in larva; or undeveloped young. These develope and take the 

 form of a Planaria, and are the Planuks of Sir J. G. Dalyell. (Fourth Rep. Brit. Assoc., 

 1834, p. 602.) From the Planule, a kind of larve, the perfect animal afterwards proceeds. 



* Van Bcncden's very elaborate Memoir is just published in the Transactions of the Royal Society of 

 Brussels. A short abstract of it is given in a late number of L'Institut, and also in the Annals and 

 Magazine of Natural History, vol. xv., p. 346 ; and these are the only sources of information with regard 

 to it which the author lias had. 



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