200 



CHAPTERS ON EVOLUTION. 



and their neighbours. Here development resembles that of the 

 Starfishes, and begins (Fig. 114, A B) with the production of an oval, 

 ciliated body, which soon acquires a digestive system. The young 

 Sea-cucumber, in the guise of what is called its " Auricularia-stage " 

 (Fig. 1 14, c), presents a cylindrical figure, with four or five bands of 

 cilia, and bears ear-like processes hence its name. Before this 

 larva is fully formed, the future Sea-cucumber commences its exist- 

 ence as a growth existing near the larval stomach. The tentacles of 



FIG. 114. DEVELOPMENT OF SEA-CUCUMBER. 



the young Holothuria soon appear (D), the ear-like projections are 

 absorbed, the Auricularia assumes a cylindrical form, and, becoming 

 the " pupa," bears a striking resemblance to a worm. When the 

 process of absorption proceeds to a further stage, the Auricularia 

 wholly disappears ; and as the new body which has been developed at 

 its expense elongates, the young Sea-cucumber form is duly evolved. 

 Such is the course of development in the sea-urchins and their 

 allies. The chronicle in question is well adapted to supplement the 

 important considerations advanced at the commencement of this 

 chapter. There is strikingly seen in the development of these animals, 

 firstly, that broad resemblance in their earliest stages which augurs 

 for a common derivation, and which proves, what their adult structure 

 teaches, that these organisms are simply so many modifications of a 

 common plan. In each and all, the first larva gives origin to a 

 second within itself, this second growth becoming the true and adult 

 form ; so that the first larva produces the new being, as it were 

 by deputy. And whilst general similarity in development thus 

 may be taken to mean community of origin, if it has any meaning 

 at all, there remains illustrated to us the second principle involved 

 in the study of development at large. The differences between the 

 early forms of these various groups are readily enough explicable on 

 the theory that adaptation and variation (acting through undetermined 

 laws of life, or through the influence of outward conditions, or through 



