THE AMERICAN LOBSTER. 201 



pilosis than the common bine crab (Oallinectes hastitt its) or common shrimp ( Crangon 

 vulgaris) in bol li 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 Avhich the larval 

 development finally assumes in any species is a compromise between several conflict- 

 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 10J inches long lays, upon the average, 11,000 eggs, each of 

 winch is about 1.9 mm. in diameter, while Calliuectes produces, according to S. I. 

 Smith (/Si), 4,500,000 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; that of 

 Calliuectes 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 top 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 bathic 

 habit has been acquired in comparatively recent times. 



