o >v' /* 



GEOGEAPHICAL DISTEIBUTION AND 



C 



[<^H. XL. 



disk shrinks up as they increase in size, and gradually dis- 

 appears, no trace of it being visible in tlie perfect animal. 



Some species of shell-bearing MoUusca lay their ego-s in a 

 sponge-like nidus, wherein the young remain enveloped for a 

 time after their birth ; and this buoyant substance floats far 

 and wide as readily as sea-weed. The young of other vivi- 

 parous tribes are often borne along entangled in sea-weed. 



so light, that, like grains of sand, they 

 by currents. Balani and Serpulse are 

 sometimes found adhering to floating cocoa-nuts, and even to 



far out at sea. It is probable, indeed 



Sometimes 



moT 



ments of pumice 



pumice 



to be a vehicle for the transport of eggs of moUusks and 

 insects and of the seeds of plants far more effective in many 

 regions than has been hitherto suspected. Mr. Bates saw 

 pieces of it floating on the river Amazon 1,200 miles from its 

 source, the nearest volcanos of the Andes. He also observed 

 other fragments 900 miles lower down the river, which in the 

 rainy season are floated at the rate of from three to five miles 

 an liour."^ Thev 



must 



miles 



currents for hundreds of 

 departure. 



In rivers and lakes, on the other hand, aquatic univalves 

 usually attach their eggs to leaves and sticks which have 

 fallen into the water, and which are liable to be swept away, 

 during floods, from tributaries to the main streams, and from 

 thence to all parts of the same basin. Particular species 

 may thus migrate during one season from the head waters 

 of the Mississippi, or any other great river, to countries bor- 

 dering the sea, at the distance of many thousand miles. An 

 illustration of the mode of attachment of these eggs will be 

 seen in the annexed cut (fig. 136). 



A lobster {Astacus marinus) was taken alive covered with 

 living mussels {Mytilus edulis) ; f and a large female crab 

 {Cancer paguriis), covered with oysters, and bearing also 

 Anomia epMppium, and Actinias, was also taken in 1832, oif 

 the English coast. The oysters, seven in number, included 



F 



"* Naturalist of the Amazons, vol. ii. p. 170. 



t The specimen is preserved in the Museum of the Zool. Soc. of Lonclou. 



I 





t 



I 



L 



SI 



it 



t: 

 t 



v. 



1 

 1 



1 



( 





