INTEOPUCTION. XV 



the hyperstroph genus Planorbis the embryonic shell is sinistral, 

 becoming dextral at a very early stage of growth, though the 

 animal remains sinistral throughout life, moreover the protoconch 

 or embryonic shell is frequently of a horny texture, even though 

 the later growth is of a calcareous nature, and in all cases the 

 line of demarcation can be plainly seen between the embryonic 

 shell and the later formed test, this line being probably caused by 

 a rest period during which the young animal was occupied in the 

 fuller development of other parts. 



The larval stage of the freshwater Pelecypoda calls for con- 

 siderable comment, so different is the embryo of the Unionidae 

 from the later stages of development that it was at first described 

 as a different creature under the name Glochidiwn, a name which, 

 though it can no longer be employed in a generic sense, is still 

 used to differentiate it from the later stages of growth. The 

 Olochidium upon its emancipation from the parent animal is a 

 helpless creature enough, being unable to swim freely or to fend 

 for itself in any way, indeed so helpless is it, that it must perforce 

 become parasitic in its habits ; so that it may attain this object, 

 it is furnished with a hook-like process in the median part of the 

 margin of each valve, which is perforate, and in addition a long 

 byssal thread ; should a fish approach, the valves are rapidly 

 opened and shut, by which action the byssal thread is thrust 

 forward, and should this touch the approaching fish it clings fast 

 to it ; thus attached it will generally, by the movements of the 

 fish, be drawn into close contact with it, when the hook-like pro- 

 jections immediately grip the intended host beyond power of 

 dislodginent ; to allay the irritation occasioned by the adherence 

 of the Glochidium the unwilling host causes a skin or cyst to 

 form over it, and thus the embryonic naiad obtains a new home 

 in which to continue its development, while obtaining its susten- 

 ance from the tissues of the fish. During this second period of 

 development considerable changes take place, both the byssus and 

 the aductor muscle are dispensed with, and in place of the latter 

 two new aductors, together with the foot, make their appearance, 

 a new shell is formed underlying the embryonic test, and finally 

 the cyst is eliminated and the animal, falling from its host, begins 

 its normal life as a free agent. 



Post-larval development. On this subject little need be said, 

 the Gastropods having passed the embryonic stage proceed to 

 develop on quite normal lines, only pausing frequently for rest 

 periods, which may be due to seasonal conditions or other Causes ; 

 generally just before these periods of rest a thickening of the 

 shell is more or less formed round the outer rim of the aperture, 

 in some species when the new growth activity commences these 

 thickenings are left and appear as either slight transverse ridges 

 or sometimes varicose costulaB on the adult test, in other species 

 upon the resumption of growth they are absorbed by the animal, 

 the material thus secreted being doubtless used in the further 

 building up of the shell ; in a few genera, such for instance as 



