CRUSTACEA. 38f> 



in the form of a Nauplius (fig. 209). This Nauplius closely resembles that 

 of the Phyllopods. The body is elongated and in addition to normal 

 Nauplius appendages is marked by six pairs of ridges the indications 

 of the future feet. The anterior antennae are as usual small ; the second 

 large and biramous, but the masticatory bristle characteristic of the 

 Phyllopods is not present. The mandibles are without a cutting blade. 

 A large upper lip and unpaired eye are present. 



The adult form is attained in the same manner as amongst the Phyllo- 

 pods after the third moult. 



MALACOSTEACA. 



Owing to the size and importance of the various forms included in 

 the Malaccstraca, greater attention has been paid to their embryology 

 than to that of any other division of the Crustacea ; and the proper 

 interpretation of their larval forms involves some of the most interest- 

 ing problems in the whole range of Embryology. 



The majority of Malacostraca pass through a more or less compli- 

 cated metamorphosis, though in the Nebaliadne, the Cumacese, some of 

 the Schizopoda, a few Decapoda (Astacus, Gecarcinus, etc.), and in the 

 Edriophthalmata, the larva on leaving the egg has nearly the form of 

 the adult. In contradistinction to the lower groups of Crustacea the 

 Nauplius form of larva is rare, though it occurs in the case of one of 

 the Schizopods (Euphausia, fig. 212), in some of the -lower forms of 

 the Decapods (Penaeus, fig. 214), and perhaps also, though this has 

 not been made out, in some of the Stomatopoda. 



In the majority of the Decapoda the larva leaves the egg in a 

 form known as the Zosea (fig. 210). This larval form is characterised 

 by the presence of a large 

 cephalo-thoracic shield usu- 

 ally armed with lateral, an- 

 terior, and dorsal spines. The 

 caudal segments are well de- 

 veloped, though without ap- 

 pendages, and the tail, which 

 functions in swimming, is 

 usually forked. The six pos- 

 terior thoracic segments are, 

 on the other hand, rudiment- 

 ary or non-existent. There 

 are seven anterior pairs of 

 appendages shewn in detail in 

 fig. 211, viz. the two pairs of FK; - 21 - ZO^EA OF THIA POLITA. (After Clans.) 

 antennae (At. I. and At. //.), mx P z - second maxillipede. 



neither of them used as swim- 

 ming organs, the mandibles without a palp (md), well-developed maxilla? 

 (two pairs, mx 1 and mx 2), and two or sometimes (Macrura) three pairs 

 of biramous natatory maxillipeds (mxp 1 and mxp 2). Two lateral 

 compound stalked eyes are present, together with a median Nauplius 



n. E. 25 



