196 



CHAPTERS ON EVOLUTION, 



a fundamental likeness in structure, beneath diversity of form ; and, 

 secondly, that such general or fundamental agreement is seen in 

 the management of their internal organs digestive system, heart, 

 nervous system, &c., and especially in what zoologists term their 

 " radial symmetry " that is, their generally rounded form arising 

 from their bodily elements, so to speak, being moulded around a 

 central point (Fig. 107), the mouth. However like these animate 

 may be in general structure, they, at the same time, present us with 

 very diverse forms. On the hypothesis of special creation, nothing 

 could appear more rational than the idea that dissimilarity of form 

 was due to the separate circumstances of their creation. But such art 

 idea overlooks at the same time their general likeness in structure ; 



and it certainly takes no account 

 and gives no explanation of the 

 singular uniformity and resem- 

 blances presented by these ani- 

 mals in early life. The general 

 likeness in question, in fact, 

 simply reiterates and strengthens 

 the evidence and conclusions 

 that the varied tribes of Star- 

 fishes, Sea-urchins, Crinoids, and 

 Sea-cucumbers have arisen from 

 a common ancestry. Let the 

 history of their development 

 prove the truth and validity of 

 this conclusion. 



Selecting a Starfish as the 

 most familiar form of the class, 

 we find its early development to 

 exhibit those stages of egg-seg- 

 mentation common to the de- 

 veloping ovum of all animals, and which have been already discussed. 

 But the special features of Starfish-development soon begin to 

 show themselves in the production of a worm-like organism, 

 utterly different from the Starfish-form, and which swims freely 

 in the sea by means of the delicate cilia or vibratile processes 

 with which the sides of its body are provided. This larva 

 possesses a digestive system, a system of water-tubes and other 

 structures ; and it would thus seem as if from the egg of the Star- 

 fish a wholly different progeny was destined to arise. So unlike 

 is the young organism to the parent, that when first discovered, it 

 was described by Sars in 1835 as a hitherto unknown form under 

 the name of Bipinnaria (Fig. no, A). In due time,, however, a 

 secondary formation begins to appear within this latter body (Fig. 



FIG. 108. SBA-CUCUMBERS. 



