394 



ANIMAL RESPIRATION 



cavity, the position of which is marked by a row of holes in the 

 sheU. The Keyhole Limpet {^Fissurella) possesses the same 

 number of gills, and the water which has traversed the gill- 

 cavity makes its way out by the hole at the top of the shell to 

 which the common name of this form is due. There are two 



interesting relatives of this 

 type which lead up to the 

 arrangement described. In 

 one of these {Emargimild) 

 there is an exit slit at the 

 front edge of the shell, while 

 in the other {Riinu/a) there 



TENTACLES 



Fig. 523. — Ormer [Haliotis) seen from above after 

 removal of shell. The gill-cavity has been opened. 



Fig. 524. — Shells of various Sea-Snails 



(seen from above) 



A, Emarginula; D, Rimula; c, Fissurella; D and E, 



successive stages in growth of Fissurella. 



is a hole placed a little way from the front (fig. 524). If the 

 development of the Keyhole Limpet is traced it is found that 

 there is first of all a slit at the front edge of the shell, then an 

 opening rather farther back, and lastly the aperture at the top 

 of the shell which characterizes the adult (fig. 524). This would 

 certainly appear to be a case of recapitulation, in which the life- 

 history of the individual epitomizes the family history, and the 

 two allied forms just described retain throughout life what were 

 no doubt stages in the evolution of Keyhole Limpets. 



Examination of a typical spiral sea-snail, such as the Whelk 

 [Bticcinuin) or the Purple- Shell {^Purpiird), shows that only one 

 gill is present, one member of the original pair having been 

 sacrificed for greater efficiency, and this gill is placed very much 

 to one side (fig. 525). Two gills probably blocked up the gill- 



