The Fauna of British India, including Ceylon and Burma / edited by A.E. Shipley, assisted by Guy A.K. Marshall. Mollusca. (Freshwater Gastropoda & Pelecypoda.) / by H.B. Preston.

Date:
1915
Catalogue details

Licence: In copyright

Credit: The Fauna of British India, including Ceylon and Burma / edited by A.E. Shipley, assisted by Guy A.K. Marshall. Mollusca. (Freshwater Gastropoda & Pelecypoda.) / by H.B. Preston. Source: Wellcome Collection.

Provider: This material has been provided by London School of Hygiene & Tropical Medicine Library & Archives Service. The original may be consulted at London School of Hygiene & Tropical Medicine Library & Archives Service.

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    I VL WOJU/C'l'ION. 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 homy texture, even though the later grow th is of a calcareous nature, and in all cases the line of demarcation can be plainly seen between the embryonic shell ami 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 Uniouidaj from the later stages of development that it was at first described as a different creature under the name Glochidium, 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 Glochidium 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 dislodgment; 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 clue 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 costulse 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