THE AMERICAN LOBSTER. 
201 
phosis than the common bine crab ( Callinectes hastatus) or common shrimp (Grangon 
vulgaris ) in both of which the larval life is prolonged? The only clew to an answer lies 
in the study of the habits and economy of these animals. The course which the larval 
development finally assumes in any species is a compromise between several conliict- 
ing paths. A wide surface distribution is necessary for the continuance of certain 
animals, but in order to secure this the larval period must be extended. On the other 
hand, a long life at the surface would be death to many species. 
Natural selection is operative at all stages of development, and is effective in 
increasing the chances of survival mainly in two distinct ways : (1) Either by increasing 
the number of ova or young produced, or (2) by shortening the path of development. 
In the latter case the number of eggs is diminished and the size of the egg increased. 
The crab and shrimp have adopted the former course and the lobster has followed 
the latter. A lobster lOi inches long lays, upon the average, 11,000 eggs, each of 
which is about 1.9 mm. in diameter, while Callinectes produces, according to S. I. 
Smith (184), 1,500,000 eggs, each having a diameter of only 0.28 mm. Thus the crab, 
though much the smaller animal, lays over four hundred times as many eggs. With 
the same number of eggs as the lobster and a long larval life, the crab could not 
survive. The lobster lives in deeper water than the crab and is probably more sensi- 
tive to changes in temperature. The larval period lasts from 5 to 8 weeks; tbat of 
Callinectes probably longer, but this is not known. Any further shortening of the 
development of the lobster would lead to a considerable reduction in the number of 
eggs, and if the metamorphosis were lost completely so that the animal left the egg in 
what now corresponds to its sixth or seventh stage the conditions of life would 
be very unfavorable for the young, on account of the sedentary habits of the adults. 
The adolescent lobsters (being thus concentrated in a relatively small area) would 
fall in vast numbers the prey of fish and Crustacea, especially to members of their 
own species, before they could establish themselves securely in their retreats along 
the rocky shores. (See Chapter xi). 
The advantage of a larval life lies in securing distribution, in this case an absolute 
necessity, over wide areas up and down the coast, and at the same time in the immediate 
transportation of the young from the shore out of reach of many enemies. This being- 
true, why, it may be asked, has the larval development been shortened at all? This 
has been brought about, in all probability, because of the general slowness which 
characterizes the whole period of development and because of the great destruction 
which is wrought upon the pelagic larvae even under the most favorable conditions. 
It is very interesting to notice, as I have already mentioned (p. 200), that abbre- 
viation in development is carried a step farther in the European species. 
It is a well-known law that a fresh water life tends to shorten the development of 
animals, and this may be due to the fact that the seasonal changes of temperature are 
far greater and more abrupt in inland waters than in the ocean. 
A life in deep water tends also to shorten development and eliminate the larval 
period. Where deep-water forms at the present day have an indirect development, it 
is possible that the problem is complicated by other conditions or that the batlric 
habit has been acquired iu comparatively recent times. 
