LIFE AT THE SURFACE OF THE SEA. 199 
forms in order to gain information as to ancestral history, 
we have carefully to distinguish between characters which 
are ancestral, and characters which are secondary or adap- 
tive, and fit the larvee for their present life. It is unneces- 
sary at present to describe the different forms which are 
recognised by naturalists. Although these differ consider- 
ably among themselves, yet, if we except the Celenterata 
and the Crustacea, the larve of all other groups, worms, 
molluscs, brachiopods, echinoderms, and so on, may be 
reduced to a common type which somewhat resembles a 
medusa, which has a flattened or concave ventral surface, 
which contains the opening of the mouth, and a dorsal dome- 
shaped surface, which is continued in front of the mouth as 
a pre-oral lobe. The alimentary canal has the form of a 
bent tube with a ventral concavity. There is present also 
either a uniform covering of cilia or definite ciliated bands, 
by means of which locomotion is effected. 
Among the Celenterata we find a still simpler type of 
larva which is called the Planula. 
These pelagic larve, then, plainly tell us of a prolonged 
period before the Cambrian rocks were formed, during which 
the earliest molluscs, crinoids, asteroids were gradually 
evolved, probably upon the shores of a continent, or upon 
the bottom, it is supposed, of friths and straits, from some 
very simple swimming forms. To what then do all these 
various facts, gleaned from the fossiliferous rocks, from 
embryology, from the structure of living forms, point ? 
How do they help us to answer our question as to the origin 
of the pelagic fauna? The end of the whole matter is, I 
think, something like this. The plant life of the ocean, so 
simple in structure and so abundant in individuals, may be 
regarded as primitive. Such primitive plants, along with a 
few forms of Protozoa, may for a long period have constituted 
the entire fauna of the sea, and even down to our own day 
they continue to constitute the most important element of 
marine life. All the higher animals which are now found 
constituting part of the pelagic fauna have developed else- 
where. Their ancestors lived on the bottom, or along the 
shore, or upon the land, where the conditions of existence 
are harder and more varied, and gave opportunity for 
