1885. J on the Fauna of the Seashore. 169 



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 become specially modified for coast life ; and in the case of the 

 echiuoderms the only possible explanation of the remarkable simi- 

 larity 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 advantage 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 

 ancestor, 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 amongst 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 witli the echinoderms through Tornaria is 

 very remarkable. Possibly Amphioxus once had a Tornaria stage, 

 and has lost it just as one species of Balanoglossus has lost it, as 

 Mr. Bateson has lately discovered. 



The littoral zone has given off colonists to the other three 

 faunal regions. The entire terrestrial fauna has sprung from 

 colonists contributed by the littoral zone. Every terrestrial verte- 

 brate bears in its early stages the gill-slits of its aquatic ancestor. 

 All organs of aerial respiration are mere modifications of apparatus 

 previously connected with aquatic respiration, excepting, perhaps, 

 in the case of Tracheata, tracheae being most likely modifications of 

 skin-glands, as appears probable from their condition in Peripatus. 

 The oldest known air-breathing animals are insects and scorpions, 

 which have lately been found in Silurian strata. Prof. Pay Lan- 

 kester believes the lungs of scorpions to be homogenous with the 

 gill-plates of Limulus. Birds were possibly originally developed 

 in connection with the seashore, and were fish-eaters like the tooth- 

 bearing Hesperornis. 



The fauna of the coast has not only given rise to the terrestrial 

 and fresh-water fauna ; it has from time to time given additions to the 

 pelagic fauna in return for having thence derived its own starting- 

 points. It has also received some of these pelagic forms back again, to 

 assume afresh littoral existence. 



The deep-sea fauna has probably been formed almost entirely 

 from the littoral, not in the remotest antiquity, but only after food 



