294 THE ANATOMY OF INVERTEBRATED AXIMALS. 



somites uncovered. The cephalic flexure is so strong as to 

 throw the ophthalmic sternum, which is very wide, completely 

 to the top of the head. The basal joints of the antennae, or 

 coxocerites, are enormous, fixed totlie surrounding parts, and 

 united b}^ their anterior extremities in the middle line below. 

 Superiorly, they seemed to have coalesced with the antennu- 

 lary sternum, so as to form a projecting wedge-shaped mass, 

 which separates the antennules from the ophthalmic sternum, 

 and causes them to appear, at first, as if they were inferior 

 to the antennae. In this genus, the basicerite, ischiocerite, 

 and merocerite are much thicker and stronger than the cor- 

 responding joints of any of the other appendages ; and in the 

 closely allied Scyllarus, the facial region of which is, on the 

 whole, similarly constructed, these joints become extremely 

 expanded and flattened, and are succeeded by no procerite. 

 In these genera the scaphocerite, or squame, usually attached 

 to the base of the antenna, is absent ; and, in Scyllarus^ there 

 is another approximation to Brachyuran structure in the ex- 

 istence of distinct orbits, formed by a lobe of the carapace, 

 which descends on the inner side of the ocular peduncle, to 

 meet the base of the antenna. No median septum is formed 

 by the rostrum, however, nor are the antennules capable of 

 being folded back into distinct chambers in any Macruran at 

 present known. 



The Ano)7iura are so completely intermediate in structure 

 between the Macrura and the Brachyura, that they need not 

 be specially noticed, except to draw attention to the singular 

 deviation from the ordinary habits and form of the higher 

 Crustaceans, presented by the Pagurid^e^ or Hermit Crabs, 

 so common on all coasts. Essentially Macruran in their or- 

 ganization, these Crustacea are distinguished from all true 

 Macrura by the uncalcified arid soft condition of the integu- 

 ment of their unsymmetrical abdomen, the appendages of 

 which are for the most part abortive, those of the sixth somite 

 being modified so as to serve as claspers. It is by means of 

 these that the Hermit Crab retains firm hold of the columella 

 of the empty gasteropad shell into which it is his habit to 

 thrust his unprotected abdomen, and, covering over his re- 

 tracted body with the enlarged chela, which takes the place of 

 an operculum, resists all attempts at forcible extraction. 



The internal structure of the Brachyiira is, on the whole, 

 similar to that of the Macrura ; but the thoracic ganglia have 

 coalesced to a much greater extent than in Astacus, forming 

 a single rounded mass. The branchiae are few, never exceed- 



