An illustration of the mode of attachment of these eggs will be seen in the annexed cut. (Fig. 100.)
The habit of some Testacea to adhere to floating wood is proved by their fixing themselves to the bottoms of ships. By this mode of conveyance Mytilus polymorphus, previously known only in the Danube and Wolga, may have been brought to the Commercial Docks in the Thames, and to Hamburgh, where the species is now domiciled. But Mr. Gray suggests that as the animal is known to have the faculty of living for a very long time out of water, it is more probable that it was brought in Russian timber, than borne uninjured through the salt water at the bottom of a vessel.[921]
A lobster (Astacus marinus) was lately taken alive covered with living mussels (Mytilus edulis)[922]; and a large female crab (Cancer pagurus), covered with oysters, and bearing also Anomia ephippium, and Actiniæ, was taken in April, 1832, off the English coast. The oysters, seven in number, include individuals of six years' growth, and the two largest are four inches long and three inches and a half broad. Both the crab and the oysters were seen alive by Mr. Robert Brown.[923]
Eggs of Freshwater mollusks.
Fig. 1. Eggs of Ampullaria ovata (a fluviatile species) fixed to a small sprig which had fallen into the water.
Fig. 2. Eggs of Planorbis albus, attached to a dead leaf lying under water.
Fig. 3. Eggs of the common Limneus (L. vulgaris), adhering to a dead stick under water.
From this example we learn the manner in which oysters may be diffused over every part of the sea where the crab wanders; and if they are at length carried to a spot where there is nothing but fine mud, the foundation of a new oyster-bank may be laid on the death of the crab. In this instance the oysters survived the crab many days, and were killed at last only by long exposure to the air.