SEA MUSSEL MYTILUS EDULIS. I 29 



On our western coast the family is likewise represented by three genera and five species. 

 Mytilus edulis occurs from the Arctic Ocean to San Francisco. Mytilus calijornianus is 

 common on the California coast. Modiolus modiolus and the straight horse mussel, 

 Modiolus rectus, are also present on the California coast. Modiolaria nigra occurs from 

 Arctic waters to San Francisco. The marine mussels are characterized by a byssus 

 which is secreted from a gland located at the base of the foot. 



The fresh-water mussels belong to the family Unionidae represented by Unio, 

 Anodonta, Quadrula, and other allied genera. They are particularly common in most 

 of the rivers of the central United States. They secrete no byssus in the adult stage. 



The phylum Mollusca comprises a great variety of forms; but there is a close re- 

 lation between all the groups, which are merely modifications of the same type. It 

 includes the chitons, Amphineura; snails, Gastropoda; mussels, clams, oysters, scallops, 

 etc., Lamellibranchia; and the nautilus, devilfishes, and squids, Cephalopoda. 



The characteristic feature of these animals is a ventral, muscular foot which usually 

 serves for locomotion, but is much modified according to habit. The body is soft and 

 moist and usually more or less covered with a shell which is generally either univalve or 

 bivalve; the shell is secreted by a glandular fold of skin called the mantle. The shell 

 often consists of three layers; an outer thick, tough portion, or periostracum ; a middle 

 prismatic layer, which is much thicker; and an inner mother-of-pearl, or nacreous layer, 

 which is sometimes brilliantly iridescent. The adult forms show no sign of segmentation 

 and the body cavity is more or less obliterated. The pericardium represents the chief 

 portion of what is left of the true body cavity. Communication between the pericardium 

 and the exterior is established through the nephridia. The respiratory organs consist 

 of gills except in a few species, chiefly terrestrial, which possess a sort of lung. It is 

 probable, also, that the mantle plays an important part in respiration. The nervous sys- 

 tem consists of three ganglionic centers with connectives located respectively in the head, 

 cerebral ganglion; in the foot, pedal ganglion; and on either side of the oesophagus, 

 visceral ganglia. Sense organs of touch, sight, smell, and equilibrium may be present 

 in the head region. In the development of the Mollusca segmentation of the egg is 

 unequal and the larva? pass through a free-swimming or trochosphere stage which is also 

 the characteristic larval stage of the Annelida. There is also a probable relationship 

 with the Polyzoa. 



In distribution, we find the Mollusca occupying, in a general way, the whole surface 

 of the earth at all latitudes and altitudes. They are found in the polar, temperate, and 

 tropical regions; in the ocean; along the seashore; on land; and in fresh- water lakes, 

 ponds, and streams. Certain snails of the suborder Stylommatophora have been found 

 in mountains at a height of 15,000 feet; abyssal mollusks have been taken from a depth 

 of 2,800 fathoms. There are pelagic species which are distributed over the surface of 

 the sea, some live on the floating seaweed, while others descend many thousands of feet 

 from the surface. It is within the Tropics, however, both on land and in the sea, that 

 the Mollusca are most abundant both in numbers and varieties. 



Protective markings of a striking nature are characteristic of many mollusks. Most 

 of the pelagic species are colorless or tinged with blue. The nudibranchs, which are 

 found on the floating sargassum weed, are beautifully marked with yellow and brown like 

 the weed itself. Other species are green or red in color, similar to the algae on which 

 they live. The shellfish which live in the great depths beyond the reach of the faintest 



