Feeding and Breathing 



45 



probably tlie production of a shell closely covered 

 so much of the surface that a simple comb-like 

 organ was developed to carry on this function 

 constantly. Such an organ exists in what we may 

 regard as little different from the primitive form in 

 certain species of Mail-shells {Chiton). Into this 

 comb -like organ (ctenidiuvi) the blood flows, and 

 throuofh the tissues of the branches absorbs the 

 oxygen from the surrounding waters. In other 

 species these simple ctenidia have degenerated until 

 no longer functional, and their place is taken more 

 efficiently by a series 

 of plate - like gills 

 encircling the body 

 Such a form of res- 

 piratory apparatus 

 may be seen in the 

 Limpet, of which a 

 figure is here given. 

 Here the letters c c c 

 show the position of 

 the encircling series 

 of gill-plates, whilst > 

 the true but aborted 

 ctenidia must be 

 sought in a pair of 

 very small yellow 

 bodies situated on 

 the sides of the neck. 

 External branchiae 

 are also developed in 

 many of the Sea-slugs, taking the form of cerata, whip- 

 like, plume-like, or foliage-like processes from the back, 



Under 

 a, foot ; h, m 



surface of Limpet 



.ntle ; c, gills ; d, mouth ; 

 c, tentacles 



