MOLLUSKS 129 
do navalis is abundant along our shores and also on the coasts of 
Europe, where it has done great damage to the woodwork of the 
dikes of Holland. 
The eggs develop within the gill cavity of the teredo and are 
cast out chiefly in May and June as actively swimming ciliated 
larve. They then develop a pair of 
relatively large shells resembling those 
of a clam, and finally eyes, and a long 
foot which enables them to crawl over 
timber into which they soon burrow. 
The Cod Clam, (Cardita borealis, 
Fig. 89), is one of the foods of the cod. 
The shell is about one inch long and 
three-quarters of an inch wide, and 
there are about 20 deep, curved, fur- 

Fig. 89; COD CLAM. 
rows which radiate outward from the 
beak of the shell. The shell is covered with a dark brown skin. 
It is common on rocky and gravelly bottoms at a depth from 30 
to 600 feet from Cape Hatteras to the Arctic, and it also occurs 
on the Pacific shores of Alaska, and on the northern coasts of 
Europe. 
MARINE MUSSELS 
Most of the mussels, or Wytilidw, are marine, although a few are 
found in fresh water. The shell is elongate and thin, and covered 
with a thick skin. The Common Edible Mussel (Mytilus edulis 
Figs. 79, 90), abounds on mud flats between tide limits and ranges 
from North Carolina to California, being also common along the 
Arctic shores and the northern coasts of Europe. The shell is 
covered with a glossy black, bluish or brown skin without radiat- 
ing ridges. The mussel attaches itself to objects by means of a 
strong yellow-colored byssus-thread which is secreted by a gland 
in the foot, and which soon hardens in the air into a tough, thread- 
like anchor rope. Great masses of mussel shells cover the shallow 
flats, anchored one to another by means of these threads. They 
can, however, leave their anchorages by casting off the threads 
from the foot, and then pushing themselves about so as to move 
into more favorable situations. They can also climb by extend- 
