i/o 



CRUSTACEA EUCARIDA DECAPODA 



CHAP. 



habit (Galathea, Fig. 116), but often go down into great depths 



(Munidopsis, Fig. 114). 



Porcellan- 

 abdomen is 



Fam. 3. 

 idae. The 



folded against the thorax, 

 and the body has a crab- 

 like form. These are 

 always littoral in habit, 

 never descending into the 

 depths. Pachychelcs in 

 the tropics, Porcella-ini 

 with numerous species in 

 all seas, P. platyclieles 

 being a common British 

 species. 



Tribe 2. Hippidea. 



The Mole -crabs have 

 the habit of burrowing in 

 FIG. 116. Dorsal view of Galatimi strigosa, x . sand, and their limbs are 



(From an original figure prepared for Professor T i TP ^ 



Weldon.) peculiarly modified into 



\ 



diin 



organs 



for this 



purpose (see Fig. 117). In other respects they are seen to be 

 closely related to the Galatheidea by the form of the carapace, 

 the condition of the abdomen, and the reduced last thoracic limbs. 

 In AUm ncit, which is found in the Mediterranean, the first 

 antennae, 1 are greatly lengthened and apposed to one another, and 

 by means of a system of interlocking hairs they form a tube 

 down which the water is sucked for respiration. The object 

 of this arrangement is to ensure a supply of clear water, filtered 

 from particles of sand, when the crab is buried beneath the 

 suHiice, on these occasions the tip of the antennal tube being 

 protruded above the surf;iee of the sand. An exactly similar 

 tube is used ]>y the true Crab Corystes cassivelaunus, which has 

 simihir burrowing habits, but here the tube is formed from the 

 second antennae and not from the first, so that the tubes in the 

 two cases afford beautiful instances of analogous or homoplastic 

 .structures between which there is no honiology (see p. 189). 



1 ('!:irstaiig, Quart. J. J//CT. ,S'c/. x]., 1897, p. 211. 



