294 THE ZOOLOGIST. 
ditions which they encounter there. Hence the origin of all hard 
shells and skeletons of marine Invertebrata, various adaptations 
for boring in sand, the adoption of the stationary fixed condition, 
and similar arrangements. Almost all the shore forms of animals, 
however inert in the adult condition, pass through in embry- 
ological 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 larve cannot be of subse- 
quent 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 larve 
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 become specially modified for coast life; and in the 
case of the echinoderms the only possible explanation of the 
remarkable similarity of the larval forms of the various groups of 
widely differing adults is that these pelagic larve represent a 
common ancestor of the group. The madreporarian corals all 
spring from a pelagic larve. 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 pelagic 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 with 
