274 CRUSTACEA. 



pairs develop very soon after. The earliest free larva of Palaemonetes vulgaris 

 sliows the rudiments of the two anterior pairs of thoracic limbs, while the 

 posterior pairs arise independently in succeeding stages (W. Faxon). 



Only after the thoracic limbs have developed do the pleopoda 

 appear as buds. By the development of the thoracic limbs into 

 biramose swimming appendages, the larva passes into the Mysis 

 stage, which varies in its appendages in the different forms, inasmuch 

 as the exopodite may be suppressed in individual thoracic limbs. 

 In Hippolijte, Caridina, and Palaemonetes vulgaris, for instance, 

 the last pair has no exopodite (this is perhaps the case in most 

 Caridid larvae), while, in Cheraphilus and Pontojihilus (G. 0. Sars, 

 IS'o. 151), and in the fresh-water form Palaemonetes varians, the 

 rudiment of the exopodite seems to be suppressed on the last three 

 thoracic limbs, in Crangon vulgaris (Ehrenbaum and G. 0. Sars) 

 and in Sahinea (Sars) on the last four. In the latter forms there 

 is thus an evident tendency to the elimination of the Mysis stage 

 from the metamorphosis. 



The tendency to simplification of the ontogenetic processes, which finds 

 expression in the above conditions, has led, in individual cases, to a much 

 more marked abbreviation of metamorphosis. The embryo of the arctic form 

 Hqypolytc polaris, as observed by Kroyeu (No. 136), showed the five pairs of 

 ambulatory limbs as simple (uniramose), jointed appendages, the rudiments of 

 pincers being already manifest on the anterior pair. Tlie five anterior pairs 

 of abdominal limbs could also be recognised as biramose rudiments, while the 

 sixth pair was still wanting. The metamorphosis of Sahinea (G. 0. Saks, 

 Xo. 151) seems similarly abbreviated, the youngest larva {Myto Gaimardii of 

 IvKOYER) hatched from the egg already showing the rudiments of all the five 

 ambulatory limbs and of the five anterior (and already biramose) pairs of 

 l)leopoda. Only the first ambulatory limbs have exopodites, the second being 

 uniramose and very small. In Sdcrocrangon boreas, the eggs of which are 

 (listingi;ished by their large size, there appears to be entire suppression of 

 metamorphosis (Spence Bate, ISTo. 100, G. O. Sars, jSTo. 151), and the same is 

 the case with the deep sea forms Cryptoclicles and Bythocaris (G. 0. Saks, 

 No. 151). 



Many of the fresh -wg,ter Caridea {e.g., Caridina Desmarestii) have, in 

 contrast to their marine relations, no abbreviation of metamorphosis, but 

 such abbreviation is distinctly evident in Falaemon Potiuna (F. Mullek, 

 No. 143) and Palaemonetes variam (P. Mayer, No. 138). Falaemon Potiuna 

 leaves the egg at a stage of development very similar to that of Hippolyte 

 2)olaris, being only distinguished from the latter by the backward condition 

 of the mouth-parts (for, owing to the larger amount of food-yolk present, the 

 larva does not commence to feed until some time after hatching), and by the 

 l)resence of gill-rudiments. 



The variety of Palaemonetes varians, wliich occurs in fresh water in the 

 South of Europe, hatches, according to P. Mayer, at an advanced Zoaea stage 

 representing the transition to tlie Mysis stage, and possesses all the limlis 

 except the last pair of pleopoda. The two anterior pairs of ambulatory limbs 



