THE MANTLE-COVERED ANIMALS. 117 
breathes through its mouth.* Look back to the 
earlier groups and you will see that the sponges, 
jelly-fish, and corals breathe through the skin, while 
the star-fish takes in water, not through his mouth, 
but through the perforated plate in his back ; the 
oyster breathes by means of gills fringing his body, 
and we shall find by and by that insects breathe 
through holes in their sides. We must look then 
for the gills of the periwinkle, and we find them 
safely lodged in a fold of his mantle over his neck, 
just within the broad part of his shell (br, Fig. 44). 
There they are bathed in water drawn in by their 
waving lashes, and when the periwinkle is left high 
and dry by the tide he pulls-to his lid, shutting in a 
supply of water. 
The same is true of the limpet, not that he has 
any door to close, but he clings so closely to the rock 
that water is shut in all round his gills, which fringe 
his body just above the foot. You would hardly 
imagine at first that a limpet has a head like a 
periwinkle, but when he is covered by the water and 
not afraid that the birds will peck at his tender foot 
and carry him off for food, you may see him lift his 
shell and put out his head with its horns, and make 
a track off to the nearest seaweed, where he grazes 
steadily. But when the tide goes down you will 
find him back again in exactly the old spot, where 
he has worn a little basin for himself to lie in, to 
which he fits so closely that sometimes his shell will 
* Exception may be taken to this generalisation as regards the 
ascidians, but it must be remembered that, so far as the true nature 
of these has been determined, they appear to be degraded members of 
the vertebrate type. 
