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On tlie other hand tlieie are numerous Decapoda, besides ciabs, in which no rostrum is pre- 

 sent ; and in some Loi-icate Macrura the eyes lie at right angles to the long axis of the body and 

 are embedded in perfect orbits, thougli these are not formed in the same way as the Brachyurous 

 orbit. 



The segments of the ceiihalothorax are immovably united together, as in 

 Nepliro])s, but the sterna of the last 5 or G segments are of great breadth. 



The relative breadth of the Brachyurous " sternum " is very variable. In the Baninido} 

 the posterior thoracic sterna are even narrower than in Nephropa. On tho other hand there are 

 many Decapods besides crabs, e.g., the Loricates, the Galatheoids, the .Porcellanoids, and the 

 Lithodoids, in which there is a broad " sternum." 



As in Neplirops, the cephalothorax is strengthened, without unnecessary 

 increase of weight, by an internal framework of ajwdemes. In most crabs in 

 addition to the transverse apodemes there is a longitudinal apodeme — the median 

 apodeme — formed by an infolding of the cuticle of some of the posterior thoracic 

 sterna in the middle line, and united posteriorly with the fu,sed and infolded 

 endosternal apodemes of the last thoracic sternum. 



In the Bromides alone there is no median apodeme ; but the inner or endo- 

 sternal apodemes of the last thoracic sternum are strongly infolded longitudinally, 

 so as to lie close together in the middle line of the body and thus to form the 

 beginnings of a median apodeme. 



The carapace of the Brachyura and its sutures and grooves will l)e con- 

 sidered subsequently. 



2. Of the Ajppendagcs of the Brachyura. 



The appendages of the cephalothorax correspond very closely with tliose of 

 Nephrons from which they differ only in certain details of form. 



The 1st pair of aj)]JCiidages of the head are the ri/es : these are borne on 

 protopodites, of which the coxal joint is usually concealed, although in some 

 crabs (e.g., Homohi) it is of considerable length and is fully exposed, and they 

 are commonly lodged in orhits. 



The simplest form of orbit (seen in the Dromiacea) is a shallow cavity into 

 which both the eye and the antennule are retractile : the most perfect form of 

 orbit is a deep pocket complete on all sides and completely separated from the 

 antennular fossee. In a considerable number of crabs the orbits are wanting. 



The 2nd pair of appendages of the head arc the antennules, which resemble 

 those of Nejihrops in composition, but differ conspicuously in the extreme short- 

 ness of their flagella, and in being as a rule retractile into pits {anteaaular fossx) 

 beneath the front. 



There are other Decapod Crustacea, besides crabs, in which the antennules are more or less 

 folded and their flagella much abbreviated. On tho other hand, there are not a few crabs in 

 which antennular fossa; are either altogether absent, or are not distinct from the orbits. 



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