296 



CRUSTACEA. 



The different Brachyuran Megalopae also show variations ir details, 

 hading to the formation of distinct genera by Dana (Mare&iia, MoiiolepU t 

 Oyllcne, Triloba). The youngest Megalopa stages of different forms vary with 

 regard to the remains still to be found of the Zoaean spines. While tin- 

 still to a considerable extent retained in Carcinus maenas (Spence Bai i . 

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

 altogether wanting in the youngest Megalopa which develops from the Metaz 



The Megalopa passes gradually through several moults into the 

 final form (Fig. 139 (T). The changes during this period, which • 

 described for Carcinus maenas by Brook (No. 106), are chiefly 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 "by 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 Pinnixa, where 

 the Mctazoaea stage, at 

 f\ m ^-v /J the last moult, givr- 



direct to a young crab, 

 the Megalopa stage being 

 entirely wanting in this 

 form (W. Faxon, No. 

 126). 



The metamorphosis of 

 some land and fresh- 

 water crabs appears ab- 

 breviated in another 

 manner. West w ooi 

 (No. 156) states that 

 the young of a specie! 

 of Gecarcinus leaves tin- 

 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 s] 

 of Gecareinus, the young hatch as Zoacae. This is also the case with other land 

 crabs (Ocypoda, Gelasimus) ; it therefore appears that, in most crabs that li- 

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

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

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



Tin- abbreviation of metamorphosis in fresh-water crabs [Trichodactylus, 



F. MOlLBR, No. 143; Dilocarciturs, Goldi, No. 129; Telphusa, Mercakti, 



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



fresh-water Decapods (e.g., Palaemonetes, Astaciis, &c). The young (Fig. HO) 



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



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



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



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



"\v the fusion characteristic of the adult. 



Fio. 140.— Youngest stage hatching from the egj; of Telj>Misa 

 fluviatilis (after Mercanti). A, dorsal aspect, ]:, lateral 

 aspect. 



