THE SPIRULA - FAMILY SPIRULIIDAE - Genus SPIRULA, Lam. Shell partially external, small, delicate, a flat, loose spiral, divided into chambers by very convex septa; lining pearly; siphuncle on ventral side of chambers; body much larger than shell; head large; eyes prominent, arms ten, short, set with sessile suckers; tentacles not dilated into clubs at the ends; mantle folds clasp and conceal shell except at two sides; posterior end of body has terminal sucker. Habitat deep water of tropical seas.
Spirula Peronii, Lam., is the sole representative of the genus. These little shells are flung upon tropical beaches by the thousands; the Gulf Stream and Japan Current carry them far north of their natural range. Yet the creature to whom this shell belongs is almost unknown. We have it from Professor Owen and Mr. Arthur Adams that the oblong body has a sucker at its posterior end by which it holds to the rocks, while its ten tacles remain free to capture food. Two backward-turning folds
of the mantle clasp the shell, covering it, except at the two oppo site points.
In any cabinet we may recognise this delicate spiral shell, wound so loosely that there is no union of the coils, but a gradual widening of the separation as growth proceeds. Thick septa divide the shell into chambers, and a siphuncle threads its way through each, but not in the centre, as in the Nautilus shell. It • keeps close to the inner wall of the coil, the floor of each chamber. If ever such an animal lived in the outer chamber of its shell, the animal was smaller or the shell larger than now. What possible use the shell serves I cannot imagine, unless to throw light on the progressive development of the cephalopods.
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