296 



CRUSTACEA. 



Tlie different Brachynran Megalo})ac also sliow variations ii details, these 

 leading to the formation of distinct genera by Daka {ilareuia. Monolcins, 

 Cyllene, Triloba). The youngest Mcgalopa stages of different forms vary witli 

 regard to the remains still to be found of the Zoaean spines. While these are 

 still to a considerable extent retained in Carcinus rnaenas (Spexce Bate, 

 No. 97), they are, in other cases, more degenerate, and may even (Portunus) be 

 altogether wanting in the youngest Megalopa which develops from the Mdazoaca. 



The Megalopa passes gradually through several moults into the 

 final form (Fig. 139 C). The changes during this period, which were 

 described for Carcinus maenas by Brook (Xo. 106), are cliiefly in 

 the shape of the dorsal shield and the degeneration of the abdomen 

 typical of the Brachyura, this part of the body being from this time 

 curved ventrally and applied to the thoracic sterna. 



While, in hj far the greater number of Brachyura, metamorphosis takes 

 the course described above, it undergoes considerable abbreviation in individual 

 cases, by the suppression of certain stages. An interesting example of this is 



found in Finnixa, where 

 the Metazoaea stage, at 

 the last moult, gives rise 

 direct to a young crab, 

 the Mcgalopa stage being 

 entirely wanting in this 

 form (W. Faxox, No. 

 126). 



The metamorphosis of 

 some land and fresh- 

 water crabs appears ab- 

 breviated in another 

 manner. West wood 

 (No. 156) states that 

 the young of a species 

 of Gemrcinus leaves the 

 egg in a condition which, apart from the want of pleopoda, altogether resembles 

 that of the adult. Thompson, on the other hand, found that, in other species 

 of Gecarcinus, the J'oung hatch as Zoacue. This is also the case with other land 

 crabs {Oci/poda, Gelasimus) ; it therefore appears that, in most crabs that live on 

 land, metamorphosis is not abbreviated, and that the young Zoaeae are hatched 

 in the sea, a fact connected with the regular migration of the land crabs to the 

 sea-shore (F. Mulleu, No. 16). 



The abbreviation of metamorphosis in fresh-water crabs (TricJiodactylus, 

 F. MtiLLER, No. 143; Dilocarcinns, (ii'iU)!, No. 129; TelpJtum, Mercanti, 

 No. 139) is, on the other hand, in agreement with what is known of other 

 fresh-water Decapods (e.g., Palaemonetcs, Astacus, &c. ). The young (Fig. 140) 

 here leave the egg in a form already greatly resembling the adult. The eyes 

 still appear comparatively large, and the cephalo-thorax, on account of the 

 presence of food-yolk, is much swollen. The abdomen has no pleopoda. In 

 Dilocarciiius, the segments of the abdomen are still quite distinct, and do not 

 yet show the fusion characteristic of the adult. 



Fig. 140. — Youngest stage hatching from the egg of Teliihiisa 

 fluviat'dis (after Mercanti). A, dorsal aspect. /-', lateral 

 aspect. 



