1 66 BULLrETIN OF THE BUREAU OF FISHERIES. 



the adult crab. The development is abbreviated in the Gegarcinus ruricola, the gaily 

 colored terrestrial crab of the West Indies, the large eggs and young of which were 

 a puzzle to the early observers. 



(4) Mysis or schizopod stage. The biramous condition of the thoracic legs char- 

 acteristic of this stage is transitoryLin_th.e-J[aryie of the higher Crustacea, but perma- 

 nent in the lower order of schizopods. The oar-like exopods of the larval thoracic 

 appendages persist in the lobster until the fourth molt, when they are suddenly reduced 

 to rudiments, and after the fifth stage no vestige of them remains. 



(5) Larval period reduced in various degrees, and metamorphosis in some cases 

 practically absent. In addition to the crayfishes, lobsters, and other illustrations of 

 abbreviated development already given, we may mention Synalpheus longicarpus of the 

 West Indies as a striking example, in addition to certain fluviatile and many deep-sea 

 forms. 



Like other animals, the Crustacea tend to recapitulate, in some degree the history of 

 their ancestors in the course of their own development, and to become modified in 

 structure and instincts to fit them for a temporary pelagic life which is totally unUke 

 that assumed when adult. Their history is further complicated, as has just been seen, 

 by the tendency to abridge the larval period or lengthen the time spent in the egg. 



Shortening the path of development is not a peculiarity of arthropods, but is 

 common with both vertebrates and invertebrates. It depends in a large degree upon 

 the relative amount of food yolk and protoplasm of the egg cell, both of which are 

 derived from the parent, and primarily upon the unknown variations and conditions 

 which have led to this result. The size of the egg is proportional to the amount of yolk 

 which it contains, not the size of the animal producing it. Thus the egg of a snapping 

 shrimp I to 2 inches long may be many times larger than that of the lobster, while the 

 egg of the latter is hundreds of times larger than that of the blue crab. When the 

 amount of yolk is small, as in the egg of the starfish or spiny lobster, the young hatch 

 in an immature condition; at the other exti;eme, when the egg is relatively large, as in 

 the crayfish or domestic fowl, the whole period of early development is passed at the 

 expense of the egg substance, and within its envelopes. The chick hatches in the form 

 and with many of the instincts of an adult bird, ripe for the experience of bird life and 

 capable of using it with profit. 



The yolk retards the progress of development up to the time of hatching, but 

 greatly shortens the adolescent period. The chick of the domestic fowl spends 21 days 

 in the egg, but in the hands of the poultry breeder it may later attain the weight of i^ 

 pounds in 3 months, when it is ready for market. * 



On the other hand, the egg of the starfish or sea urchin, which is unencumbered by 

 a great mass of yolk, and very small in consequence, measuring about ^^ inch in 

 diameter, hatches at ordinary temperatures in 24 hours. It must, however, lead a long 

 life as a larva, make its own living, run the gauntlet of enemies, and keep up the struggle 

 for months. Thus the handicap at the start may count for little in the end. The 

 advantage gained by the fowl in having a few very large eggs is offset by that of a vast 

 number of almost microscopical ova in the echinoderm. 



