MOLLUSKS 



129 



Fig. 



CDD CLAM. 



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 

 larvje. They then develop a pair of 

 relatively large shells resembling those 

 of a clam, and finally eyes, and a long 

 foot wliich enaljles 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- 

 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 llatteras 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 Mytilidcv, are marine, although a few are 

 found in fresh water. The shell is elongate and thin, and covered 

 with a thick skiu. The Common Edible i\lussel (Mytilns edulis 

 Fig>i. 7'J, 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- 



