404 Molluscous Animals of the Genus Gastrochccna. 



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tw. Case, of Gastroch^na Phobia. „/, „ Front, back, and side view, of the shell 



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iMg. 52. c represents one which had three distinct openings, 

 subsequently extended to a fourth, carried onwards in a less 

 straight direction. The enlargement, or extension, of this 

 neck may, perhaps, have been rendered indispensable by the 

 increase of the madrepores, or the accumulation of extraneous 

 substances in its vicinity, and proves how abundant is the supply 

 of the liquid provided for repairing or increasing its case. On 

 comparing the size and thickness of the secreted substance 

 with the delicacy of the shell (e,f, g), we are struck with the 

 disproportion ; at the same time, it must be supposed that 

 this covering does not commence until the animal has first 

 made its lodgement in the substance it attacked. The simple 

 perforation of a rock, or the enlargement of a previous small 

 cavity, might, perhaps, be in like manner effected by the 

 Pholades and Saxicavse, and the cell increased as more room 

 was required for its subsequent stages of growth ; but some 

 other arrangement must be provided in the condition of the 

 Gastrochse v na, if covered over in its young state by a calca- 

 reous chamber^ ?£§; these anhnals are often found in the 

 substance of old oyster shells, and not always furnished with 



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mence this envelope when the cell suited to their future 

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Fig. 52. d represents an old chamber perforated by a new 

 inhabitant. The aperture formed for its entrance was stopped 

 up from the inside, and a fresh neck issued out of the old one. 



