MEMOIRS OF TOE NATIONAL ACADEMY OF SCIENCES. 



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size and number of the eggs. These and the other facts which we have beeu considering arc 

 given in tabular view below: 



* Number not accurately determined. 



The eggs of Alphens are usually spherical when freshly laid, but they change their shape, 

 becoming more elongate in course of development and increase somewhat in size.* The eggs of 

 A. saulcyi are usually oblong. They vary from one twenty-eighth to one twenty-third of an inch, 

 taking the mean of the long and short diameters. The extreme limits of the number of eggs vary 

 somewhat from the numbers given above, which are the average limits. 



In the genus Alphens we thus have several stages in the abbreviation of the metamorphosis 

 between the macrouran zoe'a stage and the adult form. What is the cause of this gradual suppres- 

 sion of the zoe'a like form f The conclusion seems to be unavoidable that in the ISahaman species 

 this shortened life of the larva is directly related to the conditions of life. As the adults of the 

 species in question became more and more dependent upon a semiparasitict mode of life, it would 

 be clearly beneficial to reduce the larval period, in order that the young might be hatched fitted to 

 live in an environment similar to that of the adults. It the zoea brood were swept out to sea by I he 

 tides, and were to spend several weeks in the larval condition at the surface of the ocean, the 

 chances for large numbers to find particular sponges along the shores, when the. adult state was 

 reached, would be greatly lessened. It is likely that the larva? of this Alphens are never carried far 

 from the shores, but while they undoubtedly leave the sponge in which they are born, they prob- 

 ably establish themselves very soon in a new one. (The young remain a short time after batching, 

 attached to the swirnmerets of the mother.) 



This supposition is strengthened by what we know of the peculiar history ofAlphctin Itelcrochclift. 

 The Nassau heterochelis probably never changed its adult habits or adopted a parasitic mode of life ; 

 consequently it has retained undisturbed its complex larval development. The Floridian form has 

 become a parasite, and its metamorphosis is accelerated as the result. From this the I.eaufort 

 Alpheus with its less abridged development has doubtless been derived (the species extending north- 

 ward from the Gulf of Mexico), and it is within the possible, at least, to suppose that in this form 

 the metamorphosis, once lost by parasitism, is now being reestablished. 



No fewer than three species of macroura, together with the Alpheus above described, occur in 

 the large brown sponges (Illrcinia arcula) of the Bahama islands. These (one of which is also an 

 Alpheus) live in the larger oscula, are less regular in their occurrence, and evidently have not 

 adopted a stationary parasitic life. In none of them is the metamorphosis of the larva abbreviated. 

 Alpheus mi nun is also reported as occurring in the large exhalent openings of sponges at Key West, 

 but in this case we do not know, first, whether this is a fixed or only a transient habit, and 

 secondly, we know nothing of its metamorphosis under these conditions. 



Thus while in Alpbeus the abbreviated metamorphosis may be explained as an adaptation 

 to a parasitic mode of life, the question is probably often complicated by conditions which are not 

 easy to determine. There is a general tendency among the higher forms of certain groups, as in 

 the Cephalopods among the Mollusca, to reach the adult conditions rapidly by omitting some of 

 the early embryonic stages. 



* An egg of .(. saiilcyi v.ir. longicarpus, just ready to hatch (PI. XXI, Fig. 5), measures T ;'rjj by T ;; ;I inch. 

 t The Alphei which inhabit sponges are commensals rather than parasites in the strict sense. They derive pro- 

 tection from the sponge colony, and receive the benefit of the circulating currents of water which are set up within it. 



