s 



INVERTEBEATE ANIMALS. 75 



Crustaceans. The nervous system is much more con- 

 centrated forwards than in the inferior tribes. The 

 abdomen is slightly developed, and is not used to assist 

 the animal in swimming. The larval form, Zoea, long 

 considered to be a distinct animal, bears some resem- 

 blance to the adult type in the Macroura. 



Section A. — Crabs ivith mouths much narrowed in front = 

 Famille des Oxystomes (Milne Edwards). 



Group 177.— Families LEUCOSIAD.E, CORYSTID^, and 

 DORIPPID^. 



Many of the Leucosiadce are readily distinguished by 

 their convex form narrowed in front into a kind of 

 muzzle, and by their porcellanous shells smooth or 

 granulated. They are tardy animals, resting with their 

 bodies covered up to the eyes in sand, often in deep 

 water. 



The CorystidcB are placed by Dana considerably 

 higher in the series. The external antennse are long 

 and hairy. One species of Corystes inhabits British 

 seas, and is called the Masked-crab. 



Doripind(e. The two posterior pairs of legs in 

 these degraded-looking crabs are situated on the hinder 

 part of the back, and are adapted for clasping. 

 Estimated number of species : recent, 60 ; British, 6. 



H Examples of Peltarlon, Pseudo-corystes, Perse- 

 pJiona, Ilia, Ixa, &c. 



Group 178. — Family CALAPPIDiE. Nearly every species 

 in this limited group is distinguished by striking pecu- 

 liarities in form or colouring ; but very little seems to 

 be known of the life-history of any one of them. The 

 Mediterranean representatives of Calappa are called by 



