334 



LOWER INVERTEBRATES. 



third species, JV. vibex, becomes prominent. The trails of these species are common 

 on the soft mud, and frequently at the end will be found a little pellet of mud beneath 

 which the animal is hidden. All the specimens, however, do not bury 

 themselves at the retreat of the tide, as they are able to live for a con- 

 siderable time out of water. Possibly JV. ohsoleta is the more common 

 form. In this, the shell is dark ^rown, and ornamented by a net work 

 of reticulating lines. It does not thrive well where exposed to the ocean 

 Fig. in.—Nassa surf, but prefers sheltered inlets, extending in large numbers into inlets 



trivittata. ' r ' o o 



where the water is decidedly brackish. In size it reaches a length of 

 about an inch. N. trivittata is slightly smaller, and white or greenish white in color. 

 The third species, iVi vibex, is still smaller, reaching a length of half an 

 inch, and banded with ashy white and pale red, the colors being brightest 

 in the southern forms. All of the jVassw are carnivorous, drilling holes 

 through the shell of other molluscs and then feeding on the flesh. They 

 are, however, not confined to livhig objects, for they will accumulate 

 in large numbers around any decaying crab or fish, and, together with fig. 418.— Wassa 

 the amphipods, soon devour all the fleshy portions. In Europe an " *" ^'"' 

 allied species, y. reticulata is an enemy of the ojster beds, drilling through the shell 

 in a short time. They usually select the young oysters, but will destroy one three 

 years old in about eight hours. 



The egg-cases of JVassa obsoleta are among the most common of marine objects. 

 They are placed on any solid object that is handy, dead shells and the 'sand-saucer' 

 egg masses of A^a^ica being most frequently used. The capsules are curiously fluted 

 and ridged, and are crowded together without order, each 

 attached by its own pedicel. 



Purpura and its allies are by some placed in the MuricidsB, 

 by others in the Buccinidse. In Purpura the aperture is 

 wide, the spire short, the -whorls enlarging rapidly ; the col- 

 umella is flattened, and the outer lip is toothed. Purpura 

 lapilli/s, a dirty white or ashen species, is common to the 

 shores of Europe and. North America, thriving better and 

 growing to a larger size in the old world, where specimens 

 are frequently zoned with brown. On our own coast there is much variation in 

 appearance, individuals from the rocky coasts, where they are exposed 

 to the surf, having the ribs of the shell nearly smooth, while those from 

 sheltered localities have them roughened by scale-like projections. This 

 species does not range much south of Cape Cod, but north of that barrier 

 it is very common. It feeds on other animals, being especially fond of 

 the acorn barnacles {Balanus balatioides) which flourish between tides. 

 The eggs are laid in small oval capsules supported on slender stalks. 

 Each capsule contains numbers of eggs, only a few of which eventually 

 hatch, the others furnishing food for those that develop. 



Purpura patula was one of the forms which furnished the famous Tyrian purple, 

 the others belonging to the Muricidse. The animals were gathered in large numbers and 

 crushed, shells and all, in mortar-shaped holes in the rocks, two or three feet in depth. 

 From the bruised mass a liquid was obtained which was mixed with a small amount of 

 soda and diluted with several times its weight of sea- water. At first the fluid was yel- 

 low, but, after exposure for a time to the rays of the sun, it changed to purple. Then the 



Fig. 419. — Purpura lapillus. 



Fig. 420. — Egg 

 capsules of Pur- 

 pura lapillus, 

 enlarged. 



