Shield-bearing Crustacea. 323 



The type-number of segments in Crustacea is reckoned to 

 be twenty-one — bead, seven ; thorax, seven ; abdomen, seven. 

 But here the same rule holds good; namely, that wherever 

 more than one pair of appendages is found to a segment, that 

 is not a simple, but a compound ring. 



The combination of several segments to form one division — 

 as the head, for instance — results in the production of a more 

 highly-organized form, by the union of the separate nerve- 

 masses, or ganglia — which in the lower forms remain separated 

 — and thus a superior mass is formed, bearing some relation to 

 the brain in the vertebrates. 



Thus, in the crab, we find the body composed of a cephalo- 

 thorax (produced by the soldering together of the first fourteen 

 rings), and an almost rudimentary tail or abdomen. 



In this division, as might be expected, we meet with those 

 most highly-organized forms, the landTcrabs, and semi-aquatic, 

 and freshwater species, all possessing a high degree of develop- 

 ment as compared with other members of the groups to which 

 they belong. 



Although the coalescence of the nervous centres is a point 

 of so much importance, yet we must not allow it to supersede 

 more general considerations of structure and habits, when 

 treating of the Invertebrata. 



For example, we find in the classification of the Crustacea, 

 that the Maia and its allies, the triangular crabs, are placed 

 before Gonoplax, Grapsus, and Carcinus — genera which display 

 in their habits a high degree of activity and intelligence. But 

 any one acquainted with the dull, lethargic habits of the 

 triangular crabs (many of which, as Hyas and Maia, for 

 instance, remain so long sedentary, that their carapaces and 

 limbs become coated with living growths of nullipore, coral- 

 lines, and sea-weeds) cannot for a moment doubt that the 

 lively shore crab (Carcinus moenas) ranks far before them in 

 reality, however they may be placed in books. 



If you refer to the plate you will find a series of Crustacea 

 figured, not all belonging to the same order, but having a 

 strong resemblance to each other in respect to the general 

 form of their shelly covering, in allusion to which I have 

 spoken of them here as Shield-bearing Crustacea. 



They include the Phyllopoda, the Trilobita, the Poecilopoda, 

 and the Eurypterida. 



The Phyllopods are represented in our engraving by the 

 fossil genus Bithyrocaris (Fig. 9), and the recent freshwater 

 Apius in its larval and adult stages (Fig. 10 a — d). 



The Trilobites, by the curious Bohemian species of Harpes 

 ungula, from the Upper Silurian formation (Fig. 8). 



The Pcecilopods, by the two recent species of King-crabs 



