ZOOGENY. 385 



stigmata of insects ; in the Nereids the vessels project 

 upon the back from above the integument as free bran- 

 chial ramules, a formation, which is again found in the 

 nudibranchiate Gasteropods. 



2199. These branchial ramules form at first two rows 

 extending along the whole dorsal region; by degrees, 

 however, the posterior set disappear, and the cervical 

 branchiae only are left, as antetypes of the gills of fishes. 



2200. In the Mollusca the branchial vessels unite to 

 form laminae upon the ventral sides, and are already 

 surrounded by a kind of thoracic cavity, the mantle, as 

 is exemplified in the tectibranchiate Gasteropods. Here 

 again the branchiae are either a simple vascular net 

 enveloped in a fold of the mantle, or elongated into 

 filaments disposed like the teeth of a comb, or into 

 laminae, &c. 



2201. The lateral branchiae of the Nereids usually 

 emit filaments resembling feet, upon the root of which 

 the true branchiae are then placed. In the Crustacea 

 these filaments harden into actual feet provided with 

 joints. The feet are therefore nothing else than bran- 

 chial filaments, which have lost their vegetative function. 



2202. In many worms the same branchial filaments 

 are converted into hairs or bristles, which are therefore 

 none other than desiccated branchial filaments. 



2203. Even the hairs of Mammalia and the feathers 

 of Birds have been left as remnants of the original bran- 

 chial formation. 



2204. Where the branchiae have assumed the lami- 

 nated form, they are surrounded by a similarly formed 

 covering or operculum. The shells of the Mollusca are, 

 as regards their signification, none other than branchial 

 opercula, or gill-covers ; the same holds good also of the 

 gasteropodal and crustacean shells, and at bottom of 

 every calcareous and horny covering of the body. 



2205. To the same category belong also the opercula 

 of Fishes, and even their scales. Fundamentally, the 

 whole epidermis is only a product of respiration or 

 oxydation. 



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