I70 



CRUSTACEA EUCARIDA DECAPODA 



habit {Galathea, Pig. 116), but often go down into great depths 



(Munidopsis, Fig. 114). 



Fam. 3. Porcellan- 

 idae. — The abdomen is 

 folded against the thorax, 

 and the body has a crab- 

 like form. These are 

 always littoral in habit, 

 never descending into the 

 depths. Fachycheles in 

 the tropics, Porcellana 

 with numerous species in 

 all seas, P. platycheles 

 being a common British 

 species. 



Tribe 2. Hippidea. 



The Mole -crabs have 

 the habit of burrowing in 

 Fiu. 116.— Dorsal view of Galatheec strigosa, x J. sand, and their limbs are 



(Froni an original figure prepared for Professor ^■ ^ -\- r ^ • , 



Weidon.) peculiarly modified into 



digging 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 Alhuncii, which is found in the Mediterranean, the first 

 antennae,-"^ 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 

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

 protruded above the surface of the sand. An exactly similar 

 tube is used by the true Crab Corystes cassivelcmnits, which has 

 similar 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 homology (see p. 189). 



' Garstang, Quart. J. Micr. Sci. xl., 1897, p. 211. 



