THE BASKET SHELLS - DOG WHELKS - FAMILY NASSIDAE Shell small, ovate; spire elongated; base of aperture a notch or short recurved canal; columella callous; operculum horny; nucleus apical. Body with forked tail; foot long and broad; siphon long; tentacles slender, bearing eyes; radula well developed; teeth arched, serrate. A world-wide marine family, near shores of temperate warm seas. Habits active, predatory.
Genus NASSA, Lam.
Characters of the family. Over one hundred species.
These shells have their name from their resemblance to the tapering, narrow-mouthed wicker baskets used in Europe to catch fish and lobsters in. The cancellated surface of the shells suggest basketry. The mollusks are taken in considerable quantities adhering to these baskets to which they are attracted through their keen sense of smell, and their corresponding appetite for fish, dead or alive. The peasants eat the species mutabilis in Italy.
The scavenger work done by these dog whelks certainly puts them on the white list of useful mollusks. The seashore is fresh and clean because they help other scavengers to remove such wrecks of fish and other creatures as the waves wash ashore. These, if left, would become vilely offensive in a short time. No better cleanser of a polluted aquarium is known than a handful or dog whelks.
The broad foot with its forked tail and lobed front is interest ing to watch as it glides along leaving a trail in the mud. The animal finally rests under a small pellet of mud, at the end of the trail. What is intended as a mode of concealment is in reality a prominent sign board: "If your looking for the maker of this trail you '11 find him under a ball of mud at the end." 71 The Basket Shells. Dog Whelks In the aquarium the sole of the foot is often applied to the surface of the water, the shell hanging downward, a familiar position assumed by certain fresh water snails.
The Nassas are very tenacious of life. They survive for months enforced hibernation in cabinets and neglected aquarium jars, and revive with no apparent damage when restored to their natural element.
The basket shells show an astonishing tendency to variation, bringing discouragement and chaos to the systematic student who wishes to draw sharp distinctions between species. About
five hundred species have been erected already, three-fourths of which Tryon has reduced to the rank of synonyms. The American species are few.
The Channelled Basket Shell (N. fossata, Gould) is the largest species in the family. 1t is one to two inches long. The surface is sculptured with revolving ridges and grooves, which show distinct and white within the wide aperture. Longitudinal grooves cross the whorls, cancellating them finely on the spire, forming strong rounded nodules on the upper half of the body whorl. The exterior is brownish yellow and dull; interior ridged, polished, callous, bright orange. Columella excavated; lip toothed. The umbilicus leads out into a deep channel that winds around the base of the body whorl.
Habitat.— California.
The Dog Whelk (N. trivittata, Say), of our Atlantic sea board is a familiar inhabitant of sandy shores from Maine to Florida. The long spire has close spiral ridges, crossed by stronger, beaded ridges. The sinus, deep between the whorls, bears a strong top row of nodules, tinted pink or yellow, and faintly banded below.
When the tide goes out these mollusks come quickly up from under the sand, and make for the water, the broad foot holding its forked tail erect, the tentacles waving ahead, and the siphon thrust out of the notch in the shell directly above the head. The shell lies horizontally upon the body; the small operculum is hidden 'by the spire. Fine dots of purple adorn the colourless body.
The Basket Shell (N. obsoleta, Say) is eroded at the apex — a basket with its bottom badly damaged. This is a dark-coloured Nassa, brown or olive, lightened occasionally by a The Basket Shells. Dog Whelks paler band around the body whorl. The surface is spirally grooved, but faintly so, and crossed by lines of growth. The old shells are dilapidated affairs, with a sort of marine mould grown thick upon them, like an epidermis. The aperture is dark brown with white banding; the lip simple, a callus covering the columella.