Introductory 



29 



Young of Swan Mussel 



must be noted. We liave seen that tlie Snail lays 



eggs, and that when the young escape from these 



they resemble their parents in 



all respects other than size. 



The Swan Mussel retains its 



eggs until they hatch, but the 



little creatures that emerge in 



no wise resemble the parent. 



They appear to represent an 



early ancestral condition of 



the species. They have a temporary two-valved shell 



with hooked tips, with which they cling to the fins 



and tails of fishes, leading a kind of parasitical 



existence for a time ; and then they drop to the 



mud where a new and permanent shell grows within 



the valves of their cradle. This part of the Swan 



Mussel's history may be dealt with more fully later. 



Our third type of structure must be sought in the 

 sea, for it has neither land nor fresh- water repre- 

 sentative. It is the Common Sepia, one of the 

 Cuttles. Here there is no sign of an external shell, 

 and the shape is altogether difterent from either of 

 those we have already considered. The Sepia is 

 quite symmetrical in form, with a distinct head 

 surmounted by a crown of eight arms and two long 

 clubbed tentacles. In the centre of this crown is the 

 mouth, armed with a pair of horny jaws not greatly 

 unlike the beak of a parrot, which they resemble in 

 their action. Within the mouth there is a tooth- 

 ribbon like that of the Snail, but it is comparatively 

 small, because the principal work of disintegration 

 of food is done by the jaws, and the Sepia's digestive 

 powers are of a superior order. The eyes are large 



