MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 331 



sitic multiply asexual ly during the larval life, and build up complex communities, while nothing of 

 the sort occurs in those species with free larva/. 



Many similar cases might be given, but we must bear in mind that they are all very different 

 from the one now under examination. In all such cases the difference is between the larva- of two 

 distinct species, while in Alpheus we have a similar difference between the. larvir of individuals of 

 a single species. 



Among other animals it is not very unusual for certain individuals which arc placed under 

 conditions exceptionally favorable for embryonic development to be born in a more advanced stage 

 than the normal for the species, and in such cases the larval metamorphosis is abbreviated by the 

 omission of the earlier stages. 



This abridgment of the larval life is not common, but many cases are known, and if the his- 

 tory of Alpheus were simply another illustration of this process of abbreviation it would not be at 

 all anomalous, although the existence of three well marked and fixed grades of abridgment in 

 Alpheus heterodielis, in three widely-separated localities, would still be remarkable and interesting. 



The life, history of the North Carolina form of this species is more abbreviated than that of 

 the Bahama form, and the metamorphosis of the Key West form is still more shortened, but, in 

 addition to the abridgment, the different forms also present most important differences in structure 

 and in the order in which the, appendages are developed; differences which are much more funda- 

 mental and profound than the mere length of the larval life. 



The various larval forms are described with so much detail in the chapter on the metamor- 

 phosis of Alpheus that it is not necessary to-repeat them here, but the following very brief outline 

 will serve to call attention to a few of the most conspicuous features : 



As several distinct species of the genus Alpheus pass through a long metamorphosis, each 

 stage of which is almost exactly the same in all the species, we may safely assume that this is the 

 primitive or ancestral metamorphosis which was originally common to all the species. It has been 

 traced in Alp/tens minor by me at Beaufort, North Carolina, and by Mr. Herrick in a similar species 

 at New Providence. Mr. Herrick has also traced it at New Providence for Alphatx normatil and 

 Alj)lieus keterochelis. In all these forms the larva hatches from the egg in a form which is very 

 similar to Fig. 13 of 1*1. xvi, and very shortly after hatching it moults and passes into the second 

 larval stage, which is the one from which Fig. i' was drawn. This larva has all its appendages 

 fully developed and functional as far backwards as the, third pair of maxillipeds. Following these 

 are three bud like rudiments, to represent the first, second, and fifih thoracic limbs, and posterior 

 to these a, long, tapering, imperfectly-segmented abdomen, ending in a flat triangular telson. 



The locomotor organs are the plumose antenna; and the exopodites of the three pairs of max- 

 illipeds. 



After the second moult the larva passes into the third stage, which is shown in PI. xvi, Fig. 1, 

 and PI. xvn, Fig. 1. The first and fifth thoracic limbs are now functional; all the abdominal 

 somites are distinct and movable, and the uropod, or sixth abdominal appendage, has appeared, 

 and its exopedite is functional and fringed with plumose hairs, while its endopodite is rudimentary. 

 The five abdominal appendages have not yet appeared. 



The first thoracic leg, which was represented by a bud in the preceding stages, has now 

 acquired a flat, basal joint and a swimming exopodite like those of the maxillipeds, but its endo- 

 podite is rudimentary. 



The fifth thoracic limb is fully developed and is the most conspicuous peculiarity of the larva 

 at this stage of development. It has no cxopodite; its basal joint is not enlarged nor flattened, 

 and its long, cylindrical, slender shaft is prolonged at its tip into a long lance-like, hair, which 

 projects beyond the tips of the antenna-. 



After its third moult the larva passes into the fourth stage, which is shown in PI. xvnr. Fig. 3. 



The carapace now begins to extend over the eyes, and the ears make their appearance, in the 

 basal joints of the antennules. There are now five pairs of plumose locomotor exopodites, belong- 

 ing to the first, second, and third maxillipeds, and the first and second thoracic limbs. Between 

 the latter and the elongated fifth thoracic limb are buds to represent the third and fourth. The 

 telson has become narrow and elongated, and the uropods are fully developed, although there is 

 as yet no trace of the other abdominal appendages. 



