MEMOIRS OF TOE NATFONAI. ACADEMY OF SCIENCES. 



370 



size and miinbcr of the eggs. These and the other facts which we have been considerin.^ are 

 given in tabular view below: 



* Nnmber not accurately d.eterniined. 



The eggs of Alpheus 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 uumber of eggs vary 

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



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

 between the macronran zoi-a stage and the adult form. What is the cause of this gradual sujjpres- 

 sion of the zoea like form ? Tlie conclusion seems to be unavoidable that in the Bahaman 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 tlie young might be hatched titte(i to 

 live in an environment similar to that of the adults. It the zoi'-a brood were swei)t out to sea by the 

 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 Alpheus 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 hatching, 

 attached to the swimmerets of the mother.) 



This supposition is strengthened by what we know of the peculiar history of Alpheus heterochelis. 

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

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

 become a i)arasite, and its metamorphosis is accelerated as the result. From this the JSeaiifort 

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

 ward from the (lulf of Mexico), and it is within the po.ssible, at least, to suppose that in this form 

 the metamorphosis, once lost by parasitism, is now being rei'stablished. 



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

 the largo brown sponges (llirchiia arcnia) of the Haliama islands. These (one of which is also an 

 Alpheus) live in the larger osculii, are less regular in their o(!Currence, and evidently have not 

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

 Alpheus minus is also reported as occurring in the large exhalent openings of s]>onges at Key West, 

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

 secon<lly, we know nothing of its metamorphosis under tlie.se conditions. 



Thus while in Ali)lieus the abbreviated metamorphosis may bo exjilained as an .adaptation 

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

 ea.sy to determine. There is a general tendency among the higher Ibrnis of certain group.s, as in 

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

 the early embryonic stages. 



• An egg of J. Haulciji var. longicarput, just ready to hatch (PI. xxi, Pig. 5), measares jju by rfhi inch. 

 t The Alplu'i which iuh.abit spon^fs are commeuNals rather than par.iMlte.s in the strict sense. They derive pro- 

 tection from the spoujjo colony, and receive the benefit of the circulutiug currents of water which are set np within it. 



