200 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



a quadrangular plate, about two-thirds as broad as long, with an even convex margin 

 bordered with long fringing setae at its hinder end. The median spine has disappeared 

 and the long lateral spines are reduced to short, stout teeth. 



In the later adolescent stages the fringing setae of the caudal-fan become greatly 

 elongated until they nearly equal the telson in length. The adult telson is somewhat 

 spatula-shaped and about as broad as long at its base. 



THE METAMORPHOSIS OF HOMARUS GAMMARUS. 



Sars studied the first three larval stages of the European lobster in specimens 

 which he collected at the surface of the ocean. He saw enough to convince him that 

 they were at this time an easy prey to fish, swimming birds, and to ocean currents 

 which swept them into unfavorable places (175). 



At Espevaer, a fishing-place on the coast of Norway, his attention was directed to 

 large numbers of lobster larva?, which were there "packed together with an enormous 

 mass of Calanides (a kind of herring) and other species of pelagic animals, upon which 

 swarms of herring and birds were feeding." 



The abbreviation of the metamorphosis has been carried a little further in Romarus 

 gammarus than in the American species. The young of the two forms apparently agree 

 in color, but are very dissimilar in size. According to Sars, the first three larvae of 

 the European lobster measure 10, 14, and 17 to 18 mm., respectively. If these meas- 

 urements are representative, the first larva of this species is larger than the second 

 larva of Romarus americanus, and the third larva larger than the sixth stage. (See 

 table 25.) 



The color of the third larva, according to Sars, is a mixture of yellow-red or brown 

 and blue-green, and at this stage the integument has lost much of its transparency. 



The carapace, the large chelipeds, and abdomen in the first larva of the European 

 species have reached a stage of development which corresponds very nearly to the 

 second larval stage of the American form. This is best illustrated by the rostrum, 

 large chelae, and telson. The second somite of the abdomen is devoid of the median 

 spine, which, as we have seen (table 36), usually disappears in the American form 

 with the second molt. Sars says that even in the first stage the anlage of the uropods 

 can be discerned beneath the cuticle. These appendages, however, are not released 

 until after the third molt, as in our lobster. 



THE SHORTENING OF THE METAMORPHOSIS IN THE LOBSTER. 



I have discussed in my work on the development of Alpheus (94, p. 380) the 

 abbreviation of the larval period in Crustacea, and described the remarkable exam- 

 ples of this phenomenon which the study of the Alphei revealed. We will now consider 

 the case of the lobster a little more closely than it was possible to do at that time. 

 What is the cause of the suppression of the zoea stage in the metamorphosis of this 

 animal % 



We can not doubt that this is a secondary phenomenon which has appeared in 

 comparatively recent times, and that some of the immediate ancestors of the lobster 

 went through the long metamorphosis after hatching, as the majority of Decapods do 

 to-day. It is equally certain that something in the environment of these animals has 

 called forth this change. Why should the lobster be better off with a short metamor- 



