ON OTTR PRESENT KNOWLEDGE OP THE CRUSTACEA. 75 



Report on the Present State of our Knowledge of the Crustacea. — 

 Part II. On the Homologies of the Dermal Skeleton {continued). By 

 C. Spencb BatEj F.R.S. 6fc. 



[Plates II., III.] 



As in the first part of this lleport the carapace or dorsal surface of the 

 Crustacea was considered, it is now intended to examine the plastron or 

 ventral surface, aud so complete our inquiry into the form and structure of 

 the dermal skeleton, previous to a consideration of the internal viscera and 

 development of the animals of the various forms in the class. 



The head, or ccphalon, is more clearly defined in Edriophthalmous Crustacea 

 than in any other order ; but even here the somites posterior to the mandi- 

 bular ring have the dorsal surface wanting ; but a clearly defined character 

 distinctly separates them from the somites that pertain to the succeeding 

 seven, which constitute the ijereion. 



This condition is less complete in Squilla (which M. Milne-Edwards has 

 selected as being " of aU Crustacea that in -which the 21 segments of the body 

 are the most distinct "), where the posterior somites of the cephalon as well 

 as the anterior two of the pcrcion are only represented by their ventral sur- 

 faces. 



This apparent incompleteness of structure, which is due rather to an 

 economy of material, has led carcinologists to consider generally that the 

 cephalon and pereion should be treated anatomically as one portion of the 

 animal imder the general name of cephalothorax. 



Thus Dana, in writing on the " Classification of Crustacea," in his ' Report 

 on Crustacea of the United- States Exploring Expedition under Capt. Chas. 

 Wilkes, U.8.iSr.,' p. 1397, says, " In these highest species, nine segments and 

 nine pairs of appendages out of the fourteen cephalothoracic belong to the 

 senses and mouth, and only five pairs are for locomotion." 



This he has taken from the Brachyural or Macrural decapod, as being the 

 highest tj'pes of the order; but if we are to report our experiences and define 

 the names and conditions of things according as they are represented in a 

 single type or groiip, every student of any special form wlU draw his own 

 conclusions from that which he has alone closely considered, and the study of 

 Crustacea as a class in the animal kingdom must be retarded, if not mis- 

 represented. 



In studying scientifically the Crustacea as a whole, it will be found not 

 only more correct but more convenient to describe and name the several 

 parts of the animal by their homologous certainty rather than by their adapta- 

 tion to fulfil different functions which demand a variation of form with the 

 greater or less importance of their requirements. 



The seven somites that form the cephalon are most closely associated, and 

 diificult to be separated from those that follow, in the Brachyural type. This 

 circumstance appears to be largely due to the powerful character of the man- 

 dibular appendages. The great strength of these organs requires such an 

 internal development of parts that they appear to preclude the posterior 

 somites from the power of growth ; consequently they become merely sufiicient 

 to siipport appendages of a supplementary character. 



This is very apparent in the Macrural order. In Palinurus the mandibles 

 are so broad and large that their removal is almost a complete decapitation. 

 It is therefore a structural necessity that tho posterior two somites of the 

 cephalon -should be supported by those to which they are most closely 



