DECAPODA. ANOMURA. 537 



the animal in the shell it inhabits. They are only absent in the 

 Lithodidae. In the Anomura the anterior part of the pleural 

 fold of the carapace (pterygostomial region) is divided from the 

 rest by a pleural suture, and projects freely in front, a notch, 

 which receives the base of the 2nd antenna, separating it from 

 the frontal border (Fig. 315). Rostrum not united by a ridge 

 with the front of the epistome. Scaphocerite of 2nd antennae 

 often present. Third maxillipeds usually narrow. The larvae 

 hatch in the zoaea form, with 2 pairs of maxillipeds (brachyuran 

 character) and pass from this into a metazoaea stage, in which 

 the 3rd maxilliped is a functional swimming appendage, but 

 the trunk-limbs are not biramous (the mysis stage being almost 

 obliterated from the life-history). 



The differentiating characters of the tribes of the Anomura 

 are given by Borradaile as follows. 



I. End-segments of 2nd-4th trunk-legs curved and flattened. 

 1st pair styliform or sub-chelate. Tail fin not adapted for 

 swimming. Abdomen bent under thorax. Rostrum small or 

 wanting. 3rd maxillipeds without mastigobranchs. Hippidea. 



II. End-segments of 2nd-4th trunk-legs not curved or 

 flattened. 1st pair not styliform, rarely sub-chelate. 



1. 6th abdominal leg adapted for swimming (except in 

 Thalassina, where it is styliform). Abdomen symmetrical 

 with the pleura usually Avell developed. 



a. Body depressed. A pleurobranch to the last trunk- 

 leg. Often a transverse suture on the telson. Abdomen 

 more or less bent under the thorax. Galatheidae. 



b. Body compressed. No pleurobranch to the 

 last leg. No transverse suture on telson. Abdomen 

 straight. Thalassinidea. 



2. 6th abdominal limb, when present, adapted for holding 

 the body in a shell or other covering, not for swimming. 

 Abdomen nearly always asymmetrical, soft and twisted, or 

 bent under the thorax. Paguridea. 



Tribe 1. THALASSINIDEA. 



Crustacea of moderate size, burrowing in sand and mud, at various 

 depths, from the sea level to 500 faths. The last thoracic segment free 

 and articulated with the preceding and often not covered by the hinder 

 margin of the carapace. In all except the Axiidae alinea thalassinica (cf. 

 p. 514) separates the median from the lateral region of the carapace. 

 The segments of the long and often soft abdomen scarcely imbricated and 



