386 BIOLOGY 



2206. The first saccular or caecal inversion of the 

 integument, as in the Leeches, Molluscs, and Snails, is at 

 once a predominance of the tegumentary, obtained through 

 the medium of the branchial formation, whereby the 

 skin commences to become a self-substantial organ of 

 respiration. 



2207. In the Scorpions the branchiae are introverted 

 sacs or cysts, into which, however, instead of water, air 

 already enters. 



2208. This insaccation is converted in the Spiders into 

 more distinct air-cysts, which finally ramify in the higher 

 insects and become true tracheae. 



2209. Lastly, the respiratory system obtains the upper 

 hand to such an extent, that, together with internal air- 

 tubes, external branchial laminae are also developed, as in 

 the Molluscs ; but in them the tracheae obtain the pre- 

 ponderance over the blood-vessels, so that these laminae 

 dry up and become wings or fins. 



2210. The wings of Insects are branchial laminae, 

 converted into air-organs. 



2211. The wing-coverings or elytra are branchial 

 opercula, and correspond to the shells of Mollusca. 



2212. Every insect therefore must properly possess 

 four wings and two wing-coverings, of which last, however, 

 rudiments only appear to be left in the nocturnal 

 Lepidoptera. 



2213. In the higher organized animals those gills 

 only which are nearer the head are persistent, the pos- 

 terior or lateral branchiae being gradually arrested. 



2214. These lateral branchiae remain in Fishes as 

 lateral mucous openings, which constitute the lateral line. 



2215. The cervical branchiae are limited to the number 

 five, which has already begun to be established in the 

 Crustacea ; namely, at the origins or roots of the five 

 anterior pairs of feet. 



2216. The number five probably derives its origin 

 from the vegetable kingdom, and that indeed from the 

 genesis of the pinnate leaves, so that one kind of nume- 

 rical law appears to prevail with respect to this organ in 



