Page:Natural History, Mollusca.djvu/313

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301
BORERS.

than in the obscure harbour of Portpatrick, which has never been visited by a foreign craft? "[1]


Genus Pholas.

The shells of this genus are of delicate and fragile texture, and of a pure white hue, when undefiled by extraneous substances. The valves are much developed behind, gaping at both ends, but especially in front. Their exterior surface is rough, with transverse scaly ridges and furrows. A spoon-shaped process springs from beneath the beaks in each valve, directed forwards. The beaks are covered by a callosity, and there are accessory plates or valves at the back of the shell.

The animal is thick and club-shaped, with long siphonal tubes united externally into one. The foot generally is large, thick, oblique, and flat at its extremity. The burrows in this genus are never lined with a shelly coat.

Frequent mention has been made in the preceding pages of the siphonal tubes, with which many genera of Conchifera are furnished; and it has been explained that, whether apparently united in one or obviously distinct, these tubes are the external organs accessory to respiration. Even when apparently united, as in the present genus, a glance at the very tip shows that there are two openings, one of which is a little smaller than the other, and commonly this subordinate orifice diverges at a slight angle from the principal one.

The latter is the entrance, the former the exit for the water, a perpetual change of which is absolutely indispensable to the life of the animal. The

  1. Edin. New Phil. Journ. 1835.