ay, APPENDIX II 
variety known as acorn-shells. They are found here, as every- 
where, incrusting stones, wharves, shells, and other objects. The 
body of the animal is surrounded by a shell, composed of six or 
more plates, and in the shape of an irregular cone with the top 
cut off; the base of the cone is attached to the object incrusted, 
while the small end is closed by a shelly operculum which may be 
opened at will. The feathery tentacles, which are modified feet, 
are then extended and kept constantly waving. The smallest 
species, Balanus balanoides, is the commonest, and is known as the 
rock-barnacle. A large species, Coronula diadema, two inches in 
diameter and with a very thick shell, lives on the surface of whales. 
Balanus porcatus has been found fossil at Hopedale and Caribou 
Island in beds of sandy clay and coarse gravel which are exposed 
between tide-marks and extend beneath the water. 
It seems not inappropriate to include in our list two forms which 
live in pools of fresh water close to the sea; one of these is a schizo- 
pod, M ysis relicta, which also inhabits Lake Superior, Lake Michigan, 
and the lakes of northern Europe. It is so closely related to a 
certain marine form as to suggest a common origin. At Indian 
Tickle abound the “fairy shrimps,” or branchiopods, in which the 
gills or branchie are situated on the feet, the eyes are large and 
stalked, and the tail is long and slender. These shrimps are able 
to live in pools which are dry for long periods, as the eggs, when 
dried, preserve their vitality for an indefinite time. They swim 
with the back downward, and the gills are bright orange. 
