. Elements of zoology, or, Natural history of animals / ed. by Reese. Zoology. OSTRACEjE THE OYSTERS, PECTENS, &C. 409 and egress of water, and the third for the passage of the foot, which is here usually more powerful, (iv.) Cardiaceje, or Cockle tribe, in which the mantle is not only closed, but extend- ed at the respiratory apertures into tubes of greater or less length. The foot is very strong, (v.) The Inclusa, in which the mantle has only one opening for the passage of the foot; at the posterior end it is prolonged into tubes of great length, that can be extended far beyond the
. Elements of zoology, or, Natural history of animals / ed. by Reese. Zoology. OSTRACEjE THE OYSTERS, PECTENS, &C. 409 and egress of water, and the third for the passage of the foot, which is here usually more powerful, (iv.) Cardiaceje, or Cockle tribe, in which the mantle is not only closed, but extend- ed at the respiratory apertures into tubes of greater or less length. The foot is very strong, (v.) The Inclusa, in which the mantle has only one opening for the passage of the foot; at the posterior end it is prolonged into tubes of great length, that can be extended far beyond the shell, as in the common Sukns or Razor-shells. Order I.— Ostraceae. 813. The Ostraceae, of all these orders, exhibit the nearest approach to the Tunicata, both in the absence of the foot, the entirely fixed condition of the body and in the low grade of their organization in general. The shell itself is usually fixed by adhesion to other masses ; sometimes the animal is attached by a byssus; and some species, which are unattached, have a slight power of changing their place, by suddenly closing their valves, and squirting out the water that was between them. In the true Oysters, there are no teeth in the hinge, which is held together by the ligament only. Several species exist, some of which are as abundant in tropical regions as the com- mon oyster of temperate seas is on the shores of Britain. Their continued abundance, notwithstanding the large quantities constantly being consumed, is less surprising when we reflect upon their astonishing fertility; as many as 1,200,000 eggs having been detected in a single individual. The Pecteus have a hinge like that of the oysters, but differ in having the surface of the valves raised up into ribs, and in having two angular projections, com- monly termed ears, by the sides of the hinge. Many of them are very ele- gantly coloured; and they are the most active of the whole order, being en- tirely unattached, and swimming with greater quic
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