396 



ANIMAL RESPIRATION 



of the floor of the Limpet's gill-cavity shows the presence of a 

 minute orange-coloured projection on either side, and the position 



Fig. 526. — Gill-cavitles of John Knox's Limpet (a) and Common Limpet (b) enlarged, as seen from above 

 with roof removed. I, Intestinal aperture; K K, kidney apertures 



and nerve-supply of these prove them to be the vestiges of two 

 gills which ancestral Limpets no doubt possessed {fig. 526). 



How then does a Limpet 

 breathe? The answer is, 

 that it has developed new 

 gills for itself of quite a 

 fresh kind, in the form of a 

 large number of little plates 

 which grow out from the 



-Diagram of Sea-Hare [Aplysia), 

 seen from above 



I, Mouth; 2, nerve-ring with ganglia; 3, one of the 

 two ganglia on the untwisted nerve-loop; 4, gill, just in 

 front of which is seen the osphradium ; 5, opening of 

 intestine; 6, heart in pericardium; 1.0,10a, right para- 

 pod; lol), left parapod folded over back. 



Fig. 528.— Diagrammatic cross section of 

 HInd-gilled Snail (a), and longitudinal section 

 of Fore-gllled Snail (b). a, Space occupied by 

 internal organs; bb, shell; cc, mantle; d, foot; 

 e, gill in gill-cavity. 



mantle-skirt that runs right round the body (as in a Mail- 

 Shell) and overhangs the foot. These mantle-gills are very 



