50 J- R. KiNAHAN on the Britannic Species of 



dages from thence to the 14th are indiscriminately interchangeable in their 

 offices, — in some being purely masticatory, in others prehensile, or ambulatory, 

 or a mixture of all three. They vary even in the different periods of life of 

 the same animals, as Spence Bate himself has shown in his valuable paper on 

 development.* 



If there be any division in the cephalo-thorax, its natural place will appear 

 to be at the termination of the mandibular, or 4th somite, which, as far as has 

 hitherto been shown, is constant in all crustacea, even in the highly aberrant 

 cirrhipeds. 



rARTICULAR HOMOLOGIES OF CRANGON. 



In that sub-group of Decapoda which includes Crangon, the following mo- 

 dification of the general archetypal form prevails : — 



First, or ocular somite (Plate I. Md, 4 1), small, inserted as a narrow trian- 

 gular wedge between the third and fourth somites. 



Second somite concealed beneath others, and only developed inferiorly. 



Third external to first and fourth, moderately developed as a quadrilateral 

 plate on each side, and becoming confluent with fourth. 



Fourth forming great mass of carapace, prolonged anteriorly, to form inner 

 border of orbit, and expanded laterally, inclosing beneath it the fifth to ninth 

 somites, and the bases of the chelipeds, wiih their attached branchise. This, 

 from its great extent, for convenience of description, is divided into the follow- 

 ing regions, as shown in the figure: — 



/, Frontal, extremely small; rj, Gastric; h, Lateral gastric, or hepatic; 

 b\ Branchial; cd, Cardiac. 



Fifth to ninth somites, concealed beneath preceding, bearing two pairs of 

 maxillas, and three pairs of maxillipods ; the inferior portions of the somites only 

 developed. 



Tenth to fourteenth somites, also, concealed beneath carapace, forming 

 under surflice of cephalo-thorax. These bear the chelipeds and their accompa- 



* On the Development of Decapod Crustacea. By C. Spence Bate, F. L. S. Trans. Royal 

 Society of London, 1857, p. 589. The Development of the Thoracic Appendages is, however, in 

 his favour. 



