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Show 52 ON CLARSTFTCATION. thought it probable that these "pseudo-hron:al" vessels are oxtrerne modifications of organs hon1ologous w.1th the water-vesselR of the Scolecida. As M. de Quatrefages has clearly shown, it iR the perivisreral cavity with its contents that, in these animals, answers to the true blood-system of the Crustacea and Insects. The embryos of Annelids are very generally eiliaterl, anrl vibratile cilia are commonly, if not univer~m1ly, developrd in son1e part or other of their organization. In both these respects they present a most marked contrast to tho succeeding classes. Fig. 2.1). Fig. 25.-.Amphithoe, an amphipodous CrustRccan. - Lateral "i,ew (A), l ong i~ttdiJ:a! ,R.nJ .'"ertical section, detached appendages and stomach (C, D). 'I he numbers 1 to XX md1cate the appendages of the corresponding somitcs. 1·. Hostrum. t. Tel son .. lb. Labrum. st. Roof of the head, or cephalostegite. os. Oostegite. B1·. 13n1!lclJJ ru. Stoma~h opened from above (D), and viewed laterally (C). a, b, c. Different pnrt~ ofthearmatUJC, 'rJIE ARAC"IINIDA. 5:3 In the CRUSTACEA (Fig. 25), the body i , distin()"ui ·habl into a variable number of "s01nite , " or definite s O"nwnt ach of which may be, and some of which alway ar , pr~vidcd with n single pair of articulated apponclag s. Tho latt r propo 'ition i true of aU exi ting Crustacea: wheth r it also hold o·ood f b the long extinct Trilobites, i a question which V{ hay no Inenns of deciding. In rnost Crustacea, and, probably in all, one or 1nore pairs of app ndage are o modifi J a to sub rve rnauclu ~ation. A pair of ·anglia is prirnitiv ly d velop d in each omito, and the gull t passes between t\vo uc es iv pair · of cranglia, a in the Annelida. No trace of a water-vascular y..:tem, nor of any va ular sy tem sjmilar to that of the Annelida, i to be found in any Crustae <1.11. All Crustacea which po s definite r spiratory organs have branchiro, or outward proc s of the wall of th body, adapt cl for re piring air by means of water; the terr - trial Isopoda, some of which exhibit a curious ruclim ntary representation of a tracheal sy tern, forming no real exception to this rule. \Vhen they are provided with a circulatory organ, it i situated on the opposite side of the alim ntnry canal to the principal chain of ganglia of the nervou y:-:;tenl; and communicates, by valvular apertures, with t.he surrounding venous sinus-the so-called "p ricarclium." Tho Crustacea vary through such a wide range of organization that I donut if any other anatomical propo ·ition, in addition to tho 'e which I have mentioned, except the pr sence of a chitinou integun1ent and the absence of cilia, can be nunciatecl, which hall be true of all th 111 mbers of th group. It is this extreme elasticity, if I may so . peak, of th crustacean type which renders the construction of any cl finition of the Crustacea, which shall include all its mmub rs aucl exclude tho next cia. s, the ARACIINIDA, so difficult. For th SpiderR, Scorpions, 1\Iite , and Tides, which con 'titut this cia· , pos o · all the charart rs which have been jn 't stated to be con1mon to the C1·1-~stacea av one; when they are provicl d with cli tin ·t respiratory organ , in fact, the o ar not external brauclti· , adapted for Lr athing aorat cl wat r, but arc a ort of in\'olntion of the integument iu tlte fonn of tnwh al tub s, r |
