434 ARTHROPODA. 



being united behind, while their ducts remain separate. The 

 structure of the nervous system is in part dependent upon that 

 of the abdomen. In the Macrura (fig. 439, C) the ventral chain 

 consists of six ganglia in the thorax, six in the abdomen, but in 

 the Brachyura (fig. 441) these all flow together in a common mass, 

 connected with the brain by two long oasophageal commissures. 



The development of most decapods is interesting from the number of 

 larval forms. As a rule a zoea (fig. 415) is hatched from the egg ; this 

 passes next into a Mysis-stage (fig. 442) in which head, thorax, and abdo- 

 men are distinct, the thorax bearing biramous feet like those of schizo- 

 pods a proof of the origin of the simple feet from the biramous type. 

 In the crabs (Brachyura) the Mysis-stage is replaced by a Megalops (fig. 

 443), in which the abdomen is well developed but the feet have lost their 



FIG. 442. FIG. 443. 



FIG. 442. Phyllosoma larva (Mysis-stage) of Palinurus. (After Gerstacker.) A, ab- 

 domen ; C', head : T, thorax ; a and i, exopodites and endopodites of thoracic feet. 



FIG. 443. Megalops larva of Portunus. (From Lang, after Glaus.) 2, antennae ; IV- 

 VIIi\ thoracic appendages ; a 2 -a 6 , abdominal somites (a* is the seventh). 



biramous character. In some prawns (Peneus) the series is rendered more 

 complete by the appearance of a nauplius and a metanauplius with many 

 appendages, before the zoeal stage. In the crayfish and many land crabs 

 the metamorphosis has been lost, but the lobster leaves the egg in the 

 Mysis-stage. Differences may occur even in the same species; thus in the 

 European Palcemonetes varians the embryo, in the sea, leaves the egg as a 

 zoea ; in fresh water in the Mysis-stage. 



Sub Order I. MACRURA. Abdomen well developed ; antennae long ; 

 ventral nerve chain elongate ; no megalops-stage in development. 

 CARIDE A. Body compressed ; no sutures on carapace ; feet weak, ex- 

 ternal maxillipeds pediform; a large scale on the second antennae. In the 

 PENEID^E there are weak exopodites. Peneus* Sicyonia* PAL^EMO- 

 NID^E, mandibles bifid at tip. Palcemon, Alpheus,* Hippolyte,* Panda- 

 lus.* In the CRANGONID.E the mandible is simple. Crangon* Sabinea* 



