26 



MARINE AND FISHERIES 



1-2 EDWARD VII., A. 1902 



chiefly sought for and is most abundantly gathered along a belt about 200 feet broad at 

 half-tide level. 



The most favourable soil appears to be that which forms what the people call 

 mud-flats. This is composed of fine sand mixed frequently with a large proportion of 

 black mud containing organic waste matters. Such soil has originated by the attrition 

 and disintegration of rocks; the transportation of dirt and vegetable substances from 

 the adjoining land ; the decay of marine plant and animal bodies, sea weeds, shells, 

 worms, fish, &l§. The aggregation of such soil can of course take place only in sheltered 

 places, where it would not be carried away by strong tide-currents, waves and storms. 

 Hence the abundance of clams in estuaries, bays, coves, and such like situations. They 

 do occur in many places in gravelly soil, even in stony and rocky places, but rarely in 

 sufficient numbers to be of economic value, and besides they are mostly of small size. 

 The habitat also effects a distinct difference upon the external appearance of the shell. 

 Those from sandy ground have a white, chalky shell and a regular shape ; those from 

 gravel are similar in colour but are liable to be smaller and more dinged ; but those 

 taken from mud are bluer in colour, often with a brown marginal band containing an 

 oxide of iron, and are of large size. 



The natural position of the clam is with its anterior end sunk farthest in the soil 

 and its siphons pointing upwards. It is usually buried to such a depth that the siphons 

 can reach to the surface. Walking between tide marks over an area inhabited by 

 clams, one observes numerous round holes in the ground from which come spurts of 

 water occasioned by the violent closing of the clams when they feel the pressure com- 

 municated through the ground several feet in advance. Hence the name ' squirt clam.,' 



FOOD OF THE clam] Plate IV., Fig. 9. 



The structure of the clam precludes the possibility of its having rapacious habits. 

 It is not provided with ej^es wherewith to spy out its food, nor with limbs to give it 

 speed in locomotion. Neither does it possess jaws, or teeth to bite and comminute large 

 objects. It leads a sedentary, solitary life (which may account for the English name 

 ' Old Maid ' ), buried in its cramped lodgings, and depending for sustenance upon the 

 minute suspended particles that are carried to it by the sea water above. Unfavourable 

 as this mode of procuring food may seen, yet it is the one made use of by vast numbers 

 of animals, and the large size, plumpness and flavour of the flesh of the clam testify ta 

 its efficiency. To this end the clam is provided with such structural peculiarities in the 

 formation and arrangement of its organs that it comes to be most admirably adapted to 

 the conditions of its environment. The surfaces of its abdomen, gills and mantle are so 

 well supplied with cilia, disposed in such a manner and vibrating in such a direction, 

 that there can be a constant inflow of fresh sea water through the ventral branchial 

 siphon, over the gills and to the mouth. It accordingly eats constantly, perhaps rather 

 drinks constantly or at least often. One writer has suggested that the expression ' As 

 happy as a clam ' may have originated from the fact that ' it is never long between 

 drinks.' Since its food is obtained in this non-selective, mechanical fashion, it is plain 

 that particles are often carried into the mouth that are not proper food. One has to 

 bear this in mind when investigating the contents of its stomach with a view to ascer- 

 taining what it feeds upon. Sand is found in considerable abundance in its digestive 

 tract. Sometimes there are found particles which do not ordinarily belong to sea water. 

 Examination of numerous specimens will decide what constitutes the staple food of this 

 mollusk. In doing this it is best to use freshly obtained clams, otherwise much of the 

 intestinal contents will be unrecognizably digested. In many the stomach may be found 

 empty, but the intestine will be quite full and marked out in its course through the 

 light coloured reproductive gland by its dark contents. If some of this is spread out on 

 a slide and examined by the microscope it will be found to contain sand or mud with 

 microscopically small organisms and debris of larger ones. Of plants there may be 

 diatoms, desmids, filamentous algse, spores of the higher algse, fragments of vegetable 

 matter, &c. Of animals there may be Rhizopods like Amoebae and Foraminifera, Flag- 

 ellata like Euglena and the Monads, infusoria like Paramoecia, bits of sponge with spicules, 



