34 



THE CRUSTACEA 



-d.S 



f: 



Truly freshwater Decapods (apart from estuarine or brackish- 

 water species which may penetrate into fresh water) are found 



among the Natantia in the 

 family Atyidae and in several 

 genera of Palaemonidae ; of 

 the Reptantia, the Crayfishes 

 of the families Astacidae and 

 Parastacidae, the monotypic 

 Aegleidae among the Ano- 

 mura, and the Potamonidae 

 (Thelphusidae) and numerous 

 species of Grapsidae among 

 the Brachyura, are also dwel- 

 lers in fresh water. Some of 

 these are more or less am- 

 phibious in their habits, like 

 many Potamonidae, and some 

 Crayfishes are found burrow- 

 ing in the earth far from 

 streams or ponds, their bur- 

 rows reaching down to the 

 ground -water. The same is 

 reported of the marine or 

 brackish-water Thalassina. 



Truly terrestrial species 

 are found among Paguridae 

 (Birgus and Coenobita) and 

 Brachyura (Gecarcinidae), 

 and it is interesting to note 

 that these are derived not 

 from freshwater but from 

 marine types, and all (except, possibly, some Gecarcinidae) pass- 

 their early stages in the sea. 



The Sergestidae belong to the plankton, occurring at the surface 

 and descending to great depths. A few Brachyura (Planes and 

 some other Grapsidae) lead a pelagic life, clinging to driftweed, 

 floating timber, and the like. 



It is worthy of note that the deep-sea Decapods include the 

 more primitive members of each of the chief subdivisions : the 

 Aristeinae among the Penaeidea, the Hoplophoridae among the 

 Caridea, the Eryonidea among the Palinura, the Pylochelidae among 

 the Paguridea, and the Homolodromiidae among the Brachyura. 



Parasitism and commensalism in varying degrees are common. 

 The Paguridae alone, which live in the empty shells of Gasteropod 

 molluscs, present a whole series of cases of commensal association 

 with Sponges, Coelentera (Fig. 183), and Polychaete worms; 



Fio. 181. 



First Zoea-stage (after the first moult) of Cnn-'ni \u 

 maenas. a', antennule ; a", antenna ; abd, abdomen ; 

 d.s, dorsal spine of carapace (the so-called "Zoea "- 

 spine) ; /, furcate telson ; md, mandible ; inx', maxil- 

 lula ; mx", maxilla ; r, rostrum ; th', th", first and 

 second maxillipeds, biramousand natatory ; th^-tlv, 

 rudiments of three following thoracic appendages. 

 (After Faxon.) 



