HTDROZOA. 69 



of the swimming organ, while the four other ten- 

 tacles, and the lips of the polypite, are as yet 

 slightly developed. 



The development of the various appendages 

 which arise along the coenosarc of the composite 

 Hydvozoa now requires to be noticed. The same 

 hydrosoma often exhibits these in different stages 

 of evolution, so that their formation admits of 

 being studied, with more or less hope of success, 

 in specimens which may seem to have reached the 

 adult condition. 



All the lateral appendages, except the hydro- 

 thecse, appear first as simple processes of the two 

 layers of the body, and in outward form are won- 

 derfully similar, the characteristic aspect of each 

 manifesting itself as growth advances. In the 

 hydrophyllia and nectocalyces the ectoderm en- 

 larges to a much greater extent than the endoderm ; 

 in most other appendages, the relations of these 

 layers are not so disproportionate. 



As above remarked, there is but slight differ- 

 ence between hydrocysts and polypites at certain 

 stages of their development. But while the 

 hydrocysts remain closed, an opening is formed at 

 the distal extremity of each polypite, villi and 

 distinct regions soon becoming visible in its endo- 

 derm al lining. 



The tentacles, as already shown, differ as to the 

 degree of vacuolation undergone by their endo- 

 derm. The lateral threads of branched tentacles 

 " appear successively as close-set buds on one side 

 of the proximal end of the tentacle, the younger 

 buds being always developed on the proximal side 

 of the older ones." When the structure of the 



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