266 LOWER INVERTEBRATES. 



In the young stage they closely resemble the genus Avicula, even to having the 

 notch in the right valve for the byssus; but, as they grow older, the body of the shell 

 becomes more ribbon-like, its edges grow wavy, and at last the shell takes on the adult 

 characters. Several species are known, all inhabitants of the eastern seas. 



In Perna, which in general appearance resembles Avicula, the cartilage grooves are 

 several in number, arranged at right angles to the line of the hinge. In some of the 

 fossils of the tertiary age, the pearly layer lining the shell is an inch in thickness. 



The family Mytilid^e embraces the mussels, in which the two valves of the shell 

 are equal, convex, and covered with a thick epidermis. The hinge is weak, without 

 teeth, and witli the ligament internal ; the posterior muscle is large, the anterior small ; 

 the foot is cylindrical and grooved, and secretes a b3ssus. The mantle is mostly free, 

 but at the posterior end the margins unite to form a rudimentary syphon with fringed 

 margins. Most of the species are marine, but a few live in fresh water. 



J/i/ti/iifi, the typical genus, has a world-wide distribution, and is represented on the 

 northern shores of both continents by the common mussel, Jli/tilus edulis. On our east 



coast this extends as far south as the Carolinas, to 

 San Francisco on the west coast, while on the 

 eastern continent it is found in Great Britain and 

 the Mediterranean and in China and Japan. In 

 color the specimens from exposed situations are 

 dark-brown or bluish-black, while in more shel- 

 „ .„ _. ,., , ,. , tered localities one frequently finds specimens of 



FiG.2u, — J////(/Ms et/it/i.s, common mussel. i. j x 



a light, pellucid, olive-green, striped with darker, 

 or occasionally all bandiiig may be absent. These mussels grow in immense quan- 

 tities in certain situations, rooks, piles, etc., being covered with a thick matting, each 

 individual of which is aiichored by its silken, yellow byssus. In Europe these mussels, 

 as the specific name imjilies, are eaten in large quantities, but with us they form a very 

 inconsiderable portion of the diet of people living near the shore. The cause of this 

 neglect may lie in the fact that they are said to be poisonous to some people. In some 

 regions they are gathered in immense, quantities, and used. as manure. . In France the 

 natural growth is far from sufficient to supply the demand for table purposes, and 

 hence large tracts near the shore are used for their cultivation. Numerous sticks are 

 driven firmly into the bottom, and the ends which project above the surface of the 

 mud are interwoven with a wicker-work which affords an anchorage for large quantities 

 of mussels. At high tide these are covered, but at low water they are exposed, and it 

 is at this time that they are gathered for the market. 



Although the mussels are anchored by a byssus, they are not compelled to live 

 sedentary lives, for at will they can drop the byssus and move about by the aid of 

 their slender foot. They can even climb, and their method of accomplishing this is 

 interesting. The foot is moved about in the direction in which they wish to go, and 

 a byssal thread is attached. This supports the animal while the foot is again extended, 

 and another thread applied to a more distant point. By continued repetitions of this 

 operation the heavy shell is gradually lifted to the desired situation. Mytilus edulis 

 flourishes best in the zone between high and low water marks, and a little below, 

 although specimens are frequently dredged in much deeper water. 



In Mytilus the umbones of the shell are terminal, and the hinge is either toothless 

 or furnished with minute teeth, while in Modiolus the umbones are a little behind the 

 end of the shell, a distinction which is well shown in our figures. Modiolus plicatyius 



