THE EVIDENCE FROM DEVELOPMENT. 201 



both of these phases) have been at work, evolving, from the common 

 larval type, the differences of form perceptible in their present-day 

 development as well as in their adult structure. This principle 

 of adaptation is perhaps best illustrated by those cases of " direct " 

 development seen in some species of Holothurians and Starfishes, 

 &c. in which the young appear in the likeness of the parental form 

 without undergoing a metamorphosis or series of changes. In such 

 .a case, the obliteration of these changes has probably depended upon 

 causes which at present we are unable to trace ; the directly de- 

 veloped forms probably representing the later products of evolution. 

 But it is needless to remark that, on the clear evidence afforded by 

 the typical development of these animals generally, the theory of 

 their common origin is in nowise affected by the elimination, here 

 and there, of the ancestral features of the race. Perhaps the sea- 

 cucumbers and starfishes represent the most typical and least altered 

 cycles of development, whilst the sea-lilies and sea-urchins present 

 us with the results of a more modified series of changes. But, theo- 

 retically, there is little difficulty in assuming that, could we look 

 backwards in time with definite glance, we should expect to see the 

 origin of our sea-stars and their allies in a stock which, if anything, 

 approaches most nearly to the form of some primitive worm than to 

 that of any other animal form. Such a primitive form is, perhaps, best 

 outlined in the larva of the Sea-cucumber itself (Fig. 1 14, B c). Indeed, 

 the evolution of the Echinoderms from some such worm-stock is one 

 of the well-founded generalisations of modern zoology. There exist, it 

 may be added, in the develop- 

 mental history of the worms 

 themselves, certain features 

 which go far to support the 

 idea of a far-back relation- 

 ship with the sea-urchins and 

 their neighbours these latter 

 forms being apparently re- 

 moved very far from the 

 worm-stock as they present 

 themselves to our view in the 

 forms of adult and perfect 

 existence. There is a curious FIG. HS.- 



marine worm, named Balano- 



.gfossus, the larva of which, known as Tornaria, certainly approaches, 

 of all other known forms, most nearly to the youthful starfishes, 

 sea-cucumbers and their neighbours. Indeed, the young Balano- 

 glossus has been described as intermediate between the young of 

 Echinoderms and the larval forms of molluscs to be hereafter 

 ^Chapter XL) described. Balanoglossus itself, is peculiar in possess- 



