LIFE AT THE SURFACE OF THE SEA. 191 
of the largest fishes known. Thus the basking shark of the 
Atlantic may measure 30 feet in length. The eggs and 
larvee of most of our marketable marine fishes are to be 
found floating near the surface, often in great abundance, 
and I am able to show you drawings of a number of these. 
Enormous numbers of these floating eggs, and of the young 
larval fish hatched from them, are devoured by larger fish 
and other creatures. 
Ascidians—Some of the Ascidians (Salpa, Pyrosoma) 
live in the surface waters, and the curious tadpole-shaped 
Appendicularia may be captured in abundance off our coasts 
with the tow-net. It may be so abundant, especially off the 
east coast, as to discolour the ocean. The tadpole larve 
of many of the fixed Ascidians spend a portion of their exist- 
ence swimming freely at the surface. 
Mollusca.—The Mollusea are not very numerously repre- 
sented at the ocean surface, but the Pteropods, the Heteropods, 
and a limited number of Gasteropods (Atlanta, Janthina, 
Glaucus) are surface forms. Limacina and Clione form an 
important part of the food of the right whale. The pearly 
Nautilus, whose beautiful shell is so well known, occasionally 
appears on the surface. It is remarkable as being the only 
living Cephalopod that possesses an external shell serving as 
a house. Most of the extinct forms were so provided, the 
Ammonites, for example, having coiled shells divided by 
partitions into chambers. Most molluses pass through two 
larval stages. The first of these, which is called a trocho- 
sphere, is a pear-shaped form with a bent gut and a ring of 
cilia in front of the mouth. In the second stage, the head 
bears a ciliated area often lobed, which is called the 
“velum,” and the larva is called a “veliger.” Swarms of 
these veliger larve may be found at the surface in the 
neighbourhood, for example, of mussel beds. They are con- 
sumed in great numbers by the larval and post-larval fishes. 
Crustacea,—The Crustacea are represented especially by 
the Schizopoda and Copepoda, which occur in enormous 
swarms. Thus Darwin (Voyage of the Beagle, p. 16) says :— 
“In the sea around Tierra del Fuego, at no great distance 
from the land, I have seen narrow lines of water of a bright 
red colour from the number of Crustacea, which somewhat 
