334 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



ever, are not uucoinmou. There has thus been uo chance or ueed for natural selection to act along 

 the line of color. On the other baud, possibly, the Alpheus of the green sponge does retjuire color 

 protection, since the females are very sluggish during the breeding season, which extends over a 

 o-ood ijart of the year. This animal is certainly well protected against any green surface, as already 

 stated. But as will be shown, natural selection has probably nothing to do with it. The bright col- 

 oring of the tips of the claws, which only are protruded from the place of concealment, recall the 

 similarly colored beads of boring annelids which abound ou the reef, and may have a protective 

 significance. This evidence, however, is not very reliable. 



The colors of certain Crustacea, and also the color of their eggs, are known to vary greatly 

 with the surroundings. In the Alpheus parasites in the brown sponges these colors vary consid- 

 erably where the surrounding conditions are the same. However, the color of the ovarian eggs is 

 always the same as that of those already laid, and although these animals were kept for several days 

 at a time in diflereutly colored dishes, we never observed any very marked change in the color of the 

 eggs, but these ex[)eriments were not continued long enough or carefully enough to be conclusive. 

 The eggs of Alpheus hecterochelis are almost invariably of a dull olive color, while as in the case of 

 the parasite of the green sponge, about one in a hundred has bright yellow eggs. In the first case 

 at least this is possibly an instance of reversion to one of the original colors from which the green 

 was derived by natural selection. In most species of Alpheus the color of the eggs is fixed and 

 uniform, and as already suggested may have a protective significance, but in a few other cases 

 where this is not true, the color is not only variable in difterent individuals, but probably also in 

 the same individual. 



In order to explain the variations which we find in these two forms, we must assume either (1) 

 that the parasites of the green sponge are a fixed variety with distinct habits, or (2) that they repre- 

 sent individuals which have migrated from the brown sponges and adapted themselves to their 

 new surroundings, or further (3) that only those chance individuals with orange-red claws .and 

 bright-green eggs, which occasionally occur in the brown sponge, find their way to the smaller 

 green species, where they acquire great vigor and size. This last supposition is evidently untena- 

 ble. If moreover the two forms, which were at first supposed to be specifically distinct, re])resent 

 fixed varieties, we ought to find the young or at least adults of all sizes in both sponges, whereas it is 

 only in the large brown variety that any small or undersized individuals occur, while a single pair, of 

 large and tolerably uniform size, is invariably found in the exhalent chambers of the green sponges. 



These and other considerations render it probable that the second (2) proposition above stated 

 is the correct one, viz, that the parasites of the green sponges were born in the brown variety, and 

 after attaining considerable size migrated thither, where they adapted themselves at once to their 

 slightly ditterent surroundings, growing to thi-ee or four times their former size, and the females 

 acquiring bright green eggs, which become a source of pi^otectiou in their new habitat. This view 

 implies the greatest variability in color and in size of the individual, and in the color of the egg, 

 which is more remarkable from the fact that it is quite unusual in this genus. 



THE EMBRYOLOGY OF ALPHEUS. 



At my suggestion Mr. Herrick undertook, in 1886, the study of the embryology of Alpheus, 

 and devoted a considerable part of bis time for three years to this subject, and while he carried on 

 the work under my general supervision the results which be has reached are entirely bis own, and 

 my share in the chapter which is devoted to this division of the subject is only that of an instructor. 

 I must call attention, however, to the fact that Mr. Merrick's studies were begun at a time when 

 our knowledge of the embryology of the higher Crustacea was far less complete than it is at the 

 present time. From time to time brief abstracts of tbe progress of the research have been written 

 by Mr. Herrick and published in the Johns Hopkins University circulars, and the following cor- 

 rected summary of his results contains the substance of these preliminary reports. 



The work was begun at Beaufort, North Carolina, and the eggs of the two species of Alpheus 

 which occur there were carefully examined and preserved for laboratory research, but much better 

 and simpler material was afterwards obtained at the Bahama Islands, the early stages were much 

 more thoroughly studied, and the development of the animals was traced in detail, step by step, 



