624 THE POPULAR SCIENCE MONTHLY. 



for respiration, and where constant variation of conditions is produced 

 by the tides, that all the main groups of the animal kingdom first came 

 into existence ; and here also, probably, where the first attached and 

 branching plants were developed, thus establishing a supply of food 

 for the colonization of the region by animals. 



The animals inhabiting the littoral zone are most variously modi- 

 fied, to enable them to withstand the peculiar physical conditions which 

 they encounter there. Hence the origin of all hard shells and skele- 

 tons of marine invertebrata, various adaptations for boring in sand, 

 the adoption of the stationary fixed condition, and similar arrange- 

 ments. Almost all the shore forms of animals, however inert in the 

 adult condition, pass through, in embryological development, free- 

 swimming larval stages which are closely alike in form for very widely 

 different groups of animals. Thus the oyster and. most other mollusca 

 of all varieties and shapes when adult, develop from a free-swimming 

 pelagic trochosphere larva, and so do many annelids. Such larva? can 

 not be of subsequent origin to the adults of which they are phases. If 

 such were the case, they would not have become so closely alike in 

 structure. In reality they represent the common ancestors from which 

 all the forms in which they occur were derived, and, as all these larvae 

 are pelagic in habits and structure, it follows that the inhabitants of 

 the shores were derived from pelagic ancestors. The earliest plants 

 were also probably free-swimming. 



In the case of the cirripedia there can be no doubt, from the history 

 of their development, that they were originally pelagic, and have be- 

 come specially modified for coast-life ; and in the case of the echino- 

 derms the only possible explanation of the remarkable similarity of 

 the larval forms of the various groups of widely differing adults is 

 that these pelagic larvae represent a common ancestor of the group. 

 The madreporarian corals all spring from a pelagic larva. The colonial 

 forms probably owe their origin and that of their skeletons to the ad- 

 vantage gained by them in the formation of reefs, and the increase in 

 facilities of respiration consequent on the production of surf. In the 

 deep sea they are very scarce. 



The vertebrata are sprung from a very simple free-swimming an- 

 cestor, as shown by the ciliated gastrula stage of Amphioxus. The 

 ascidians afford another evident instance of the extreme modification 

 of pelagic forms for littoral existence. 



The peculiar mode of respiration of vertebrata by means of gill- 

 slits occurs in no other animal group except in Balanoglossus, which 

 will probably shortly be included among vertebrata. Possibly gill- 

 slits as a respiratory apparatus first arose in a littoral form, such as 

 Balanoglossus, and hence their presence at the anterior end of the 

 body, that nearest to the surface in an animal buried in sand. The 

 connection of Balanoglossus with the echinoderms through Tornaria 

 is very remarkable. Possibly Amphioxus once had a Tornaria stage, 



