OLAM FISHERY OF PAS8AMAQU0DDY BAY 



29 



SESSIONAL PAPER No. 22a 



perhaps millions, of spermatozoa. Only one sperm-cell is necessary for the impregnation 

 of an egg. Judging by comparison with well known cases we have a right to conclude 

 that, considering the sexes to be equally abundant, the great surplus of sperm-cells for 

 each egg indicates the chances that each egg runs of failing to become fertilized. If it 

 takes a million spermatozoa to insure the fertilization of one egg, then the egg must be 

 subject to very unfavourable conditions. Nature has met these adverse conditions by 

 increasing the number of chances, so that, where currents of water or other causes 

 interfere, yet a sufficient number of eggs become impregnated to keep up the average 

 number of individuals from year to year by developing new broods to take the place of 

 those removed by accident, natural death, &c. When a sperm-cell has found an egg- 

 cell it forces its way, head foremost, by violently flapping its tail, through the outer 

 membrane. Having once gained entrance, it soon ceases to exist as a distinct organism 

 and becomes absorbed into the protoplasm of the egg, which, in consequence, now 

 assumes renewed vigour. The egg-cell soon divides into two cells, these into four, and 

 so on until a considerable number of cells is formed. During the process of cell multi- 

 plication and hand in hand with it, the cells arrange themselves in such order and 

 become modified in such ways that, in a short time, a free-swimming embryo results. 

 This is so small as to be scarcely visible to the unaided eye. It differs from the adult 

 in a number of respects, but perhaps the most important of these is its ability to swim 

 freely in the sea-water. This is accomplished by means of a peculiar organ called the 

 velum, which can be protruded from between its already formed tiny shells. — Fig. 8. 

 The velum is well supplied with large cilia, arranged in a wheel like manner. This 

 stage in the development of the clam is of great importance, for it is due to it that the 

 clams are capable of becoming scattered so that some of them may find fortunate places, 

 a? well as become distributed in entirely new regions although of course not at 

 once over great distances. After a time the young clam becomes too heavy to swim, 

 settles upon sea-weed, stones, sand, mud, &c., entirely looses its velum, but remains 

 capable of actively creeping about by means of its foot. At this period it may be less 

 than -g^Q- of an inch in length. Upon finding a satisfactory situation, it sooner or later 

 buries itself in the sand or mud and begins life after the fashion of its adult parents. 

 In the paper already mentioned Mead wrote : ' By the first week in July, 1899, a great 

 many clams had already found their way into the sand. At this time they were so 

 small that they escaped general notice, ranging from a size at which they were hardly 

 visible to 9 mm. in length. He performed a number of experiments in planting small 

 clams with a view to finding out their rate of growth. Those at extreme low tide grew 

 the most, while the rate of growth fell ofi" in proportion to the height above that level. 

 Thus a specimen 15 mm. long on July 22 was planted at low water, and on September 

 18 it measured 48 mm. Another 13 mm. long grew in the same time to 28 mm. 

 when planted below half-tide mark. Proper precautions were taken to guard against 

 error and a large number of experiments employed, with the result that they grew in 

 two months to twice, three times, and in some cases four times their original length.' 

 Another way* in which their rate of growth was measured was this : On July 6 and 

 9 a pint and a half of small clams were planted in a box of sand. On September 

 18 4 J quarts of clams were taken from half the box. This is an increase of six 

 times their bulk in 10 weeks. The same observer found at the beginning of the breed- 

 ing season a ripe male 30 mm. in length, and a ripe female 50 ram. in length. In his 

 experiments he raised clams over 30 mm. in length that were undoubtedly of that year's 

 growth. It seems likely then that clams may become mature and reproduce when one 

 year old, although it has been generally thought that they require three years to grow 

 to sexual maturity. 



ENEMIES OF THE CLAM. 



Clams, although ordinarily buried out of sight, and consequently escaping the open, 

 direct struggle that their relatives the mussels are subject to, are nevertheless preyed 

 upon by a considerable number of animals. They may be exposed through the washing 

 away of sand by storms, when they may be cast up on shore, or left to die in the sun, 



