Xiv

XIV, The first metasomatic somite, that of the mesosoma. It was (After Bnner. Ztititk. ma. Zaol yot. lu., described by Newport as " the diaphragm. Only the central and horizontal parts of this structure correspond precisely to the entosterniteof Limulus : the right and left anterior processes (marked ap in figs. 3 and 4; and RAP, LAP, in figs, i and a) correspond in the two animals, and the median lateral process Imp of the scorpion represents the tendinous outgrowths ALR, PLR of Limulus. The scorpion's FIG. 18. — Portion of a similar embryo at a later Mace of Limulus in "a of growth. The praegenital somite, VII PrG, is still present, but has lost its rudimentary appendages; go, the genital operculum, 't half; Km. the left cntosternite give* riae to outgrowth*, beside* the great posterior flap*, pf. which form the diaphragm, unrepresented in Limului. These are a ventral arch forming a neural canal through which the great nerve cord* pas* (fig*. 3 and 4, snpi. and further a dorsal gastric canal and arterial canal which transmit the alimentary tract and the dorsal artery respectively (fig*. 3 and 4. GC, DR). In Limului small entonternite* are found in each tomite of the appendage-bearing mcKMoma, and we tin.

I in Scorpio, in the only somite of the mesosoma which has a welldeveloped pair of appendages, that of the pectens, a small entosternite with ten pairs of muscles inserted into it. The supra-pectinal entosternite lie* ventral to the nerve cord*. In Mygale (figs. 5 and 6) the form of the entosternite is more like that of Limulus than is that of Scorpio. The anterior notch Ph.N. is similar to that in Limulus, whilst the imbricate triangular pieces of the posterior median region resemble the similarlyplaced structures striking manner. It must be confessed that we are singularly ignorant as to the functional significance of these remarkable organs — the entosternites. Their movement in an upward or downward direction pecten; abp' to abv, the in Limulus and Mygale must exert a rudimentary appendages of pumping action on the blood con- the lung-sacs, tained in the dorsal arteries and the (After Brucr. Itc. di.) ventral veins respectively.

In Scorpio the completion of the horizontal plate by oblique flaps, so as to form an actual diaphragm shutting off the cavity of the prosoma from the rest of the body, possibly gives to the organs contained in the anterior chamber a physiological advantage in respect of the supply of arterial blood and its separation from the venous blood of the mesosoma. Possibly the movement of the diaphragm may determine the passage of air into or out of the lung-sacs. Muscular fibres connected with the suctorial pharynx are in Limulus inserted into the entosternite, and the activity of the two organs may be correlated. 5. The Blood and the Bloodvascular System.— The blood fluids ^'V of Limulus and Scorpio are very *£*?":• similar.

Not only are the blood divisions in the regmn of the corpuscles of Limulus more like ""segmented ant.er?or carapacein form and granulation to those Th* seventh, VII. is anterior t. of Scorpio than to those of any the genital operculum. op, and rmsf^n ™r thp fluid is in IS the cavity of the praegenital p « -Section through an of L,mul»,T<mp. g seven transverse Crustacean, but the fluid is in both animals strongly impreg- 8om nated with the bfue-coloured completely suppressed in « respiratory proteid, haemocyanin. sequent development, possibly This body occurs also in the blood !?d,ic?te? by of Crustacea and of Molluscs, but vn its abundance in both Limulus entopophyses of the prosomatic and Scorpio is very marked, and a gives to the freshly-shed blood a 'Ai^Li , ,££*) strong indigo-blue tint. The great dorsal contractile vessel or " heart " of Limulus is closely similar to that of Scorpio ; its ostia or incurrent orifices are FIG. 20. — View of the ventral surface of the mid-line of the prosomatic region of Limulus polyphemus. The coxae of the five pairs of limbs following the chelicerae were arranged in a scries on each side between the mouth, M, and the metasternites, mels. sf. The sub-frontal median sclerite.

Ch, The chelicerae. ram. The camerostome or upper lip. M, The mouth. pmsl. The promesosternal sclerite or chitinous plate, unpaired. mtls, The right and left metastcrnites (corresponding to the similarly placed pentagonal stcrnitc of Scorpio). Natural size. (After Lankmter.) placed in the same somites as those of Scorpio, but there is one additional posterior pair. The origin of the pai™*! arteries from the fmsl _