228 



CHAPTERS ON EVOLUTION. 



" rudimentary organs," which, like the teeth of the unborn whalebone 

 whale possessing no teeth whatever in its adult state have a refer- 

 ence to a past state of things. These teeth and the rudimentary 

 shell are heritages derived from ancestors which had well-developed 

 teeth and shells respectively. Otherwise, and on any other theory of 

 nature, their mere existence is a hopeless and insoluble puzzle. The 

 shell-less univalves to which we refer are often familiarly named " sea 

 slugs," " sea-lemons," and the like. By naturalists they are placed in 

 such genera as Doris (Fig. 149), sEolis (Fig. 150), Aplysia, &c. Other 

 examples of these molluscs are included in the genus Bulla, or that 

 of the " Bubbleshells " (Fig. 151), possessing a delicate shell, and 

 Aplysia, or that of the " Sea-hares," famed of old as an ingredient 

 in classical poison-cups. Bulla and the Sea-hares possess each a 

 thin shell, which, however, is a secondary growth, and does not 

 represent the true shell or that developed in early life. Now, in 

 these " naked " gasteropods, there is a well-marked " veliger-stage." 

 Moreover, if the development of such a form as sEolis (Fig. 150), or 

 its neighbours of the " Sea-lemon " tribe, be studied, the young form 

 is observed at one stage of its career to present a singular and highly 

 characteristic appearance. It possesses a velum, consisting of two 

 well-marked lobes (Fig. 152, A), richly ciliated, and by means of which 

 it swims rapidly through the sea, whilst the animal's foot and its 

 shell are also readily observable. 



FJG. 149. DORIS. 



FIG. 151. BULLA. 



FIG. 152. YOUNG OF JEoLis AND ADULT PTEROPCD. 



FIG. 150. J 



Far away in the Northern seas, the Arctic voyager may sometimes 

 sail for days, or rather for nights, through water which may be 

 discoloured by the innumerable myriads of small organisms floating 

 on its surface. Each of these beings is of very small size certainly 

 under an inch in length as a maximum measurement ; and each 



