524
ARACHNIDA
takes place. Owing to its position it is convenient to term somites, regardless of its post-anal position and mode of the somite which is excalated in Limulus and Scorpio development! The agreement of the grouping of the “ the prsegenital somite.” It appears not improbable that somites, of the form of the parapodia (appendages, limbs) the sternal plates wedged in between the last pair of legs in each region, ot the position of the in both Scorpio and Limulus, viz., the pentagonal sternite genital aperture of Scorpio (Fig. 10) and the chilariaof Limulus (see Figs. and operculum, of 13 and 20), may in part represent in the adult the sternum the position and of the excalated prsegenital somite. This has not been character of the demonstrated by an actual following out of the developeyes, and of the ment, but the position of these pieces and the fact that powerful post-anal they are (in Limulus) supplied spines not seen in by an independent segmental other Arthropods, nerve, favours the view that is very convincing they may comprise the sternal as to the affinity area of the vanished prsegenital This interpretation, of Limulus and somite. Scorpio. Perhaps however, of the “ metasternites ” the most important of Limulus and Scorpio is opgeneral agreement posed by the coexistence in of Scorpio com- Thelyphonus (Figs. 55, 57, and pared with Limu- 58) of a similar metasternite similar is.-Portion lus and the Eury- with a complete prsegenital Fig. embryo at - of a stage growth. The prsegenital pterines is the somite. Hansen (10) has re- of YIIPrG, is still present, division of the cognized that the “prsegenital somite, but lias lost its rudimentary go, the genital body into the three somite ” persists in a rudiment- appendages; operculum, left half; Km, the ary condition, forming a “waist” left pecten; a6p4 to abpt, the , „ . ... regions (or tagFig. 16.—Diagram to show the way m which an ° ° rudimentary appendages of the outgrowing gill-process bearing blood-holding mata) prOSOma, to the series of somites in the lung-sacs. (After Brauer. loc. lamellae, may give rise, if the sternal body wall n „ flnfi Pedipalpi and Aranese. The cit.) sinks inwards, to a lung-chamber with air-hold- meSOSOma, anu ing lamellae. I is the embryonic condition; 6s, metaSOma each present writer is of opinion that blood sinus; L is the condition of out-growth • ,• • with gl, gill lamellae; A is the condition of m- Consisting Oif sia. it will be found most convenient to treat this evanescent sinking of the sternal surface and consequent ggcrixieilts the DrO- somite as something special, and not to attempt to reckon enclosure of the lamelligerous surface of the & ’ . appendage in a chamber with narrow orifice—the SOUia having leg- it to either the prosoma or the mesosoma. These will then pulmonary air-holding chamber; pi, pulmonary iJ1Ke
- i
a rmpn d a CPS remain as typically composed each of six appendage-bearing lamellae; 6s, blood sinus. (After Kingsley.) appendages, the mesosoma somites—the1 prosoma comprising in addition the ocular having foliaceous appendages, and the metasoma being prosthomere. When the prsegenital somite ori traces of it are present it should not be called “ the seventh prosomatic destitute of appendages. In 1893, some years after the identification of the somites or the “first mesosomatic,” but simply the “prsegenital somite.” The first segment of the mesosoma of Scorpio of Limulus with those of and Limulus thus remains the first segment, and can be Scorpio, thus indicated, identified as such throughhad been published, zooloout the Eu - arachnida, gists were startled by the carrying as it always does discovery by a Japanese the genital apertures. But zoologist, Mr Kishinouye it is necessary to remem(8) of a seventh prosober, in the light of recent matic somite in the emdiscoveries, that the sixth bryo of Limulus longiprosomatic pair of appendspina. This was seen in PrGa5pi ages is carried on the longitudinal sections, as a&p2 -op 9app seventh somite of the a6p3 shown in Fig. 19. The «6p4 senes, there being Section through an early em- whole series, Fig. 19. „ simple identification of a6p56 — bryo of Limulus^ longispina,^ showing f-L w somite with somite in seven trails verse divisions in the region w IJ0_ p r O S t h O m e r 6 S or «6p of the unsegmented anterior carapace, somites in front Ot the ap&7 Limulus and Scorpio The seventh, VII, is anterior to the m first on r ran noseemed to be threatened genital operculum, op, and is the cavity nioutn, the first carrying -n j. • Fig. 17.—Embryo of Scorpion, ventral of the praegenital somite which is more |} yes, the second the 1C e by this discovery. rmi in Yiew showing somites and appendages. or less completely in sub- chelicerse . ^ J also . ,i sequent development,suppressed possibly indicated tna t, tile 1896 Dr August Brauer sgc, frontal groove; sa, rudiment of lateral eyes; obi, camerostome (upper by the area marked VII. in Fig. 7 and g ^. tic or geni1 rg mesosoma of Marburg (9) discovered lip); so, sense-organ of Patten. PrGa&p , by the great entopophyses of the proso. . ° . of the appendage of the praegenimatic carapace. (After Kishinouye, tal SOHllte IS not the in the embryo of Scorpio a rudiment tal somite which disappears; a6p2, rudiJourn. Soi. Coll. Japan, vol. v. 1892.) seventh or even tlle eiglltll seventh prosomatic somite ment of the right half of the genital
- 1abp3, rudiment of the right of the whole series of somites which have been histori(see YII.PrG, Figs. 17 operculum
pecten; abp to abpl, rudiments of the and 18), or, if we please so four appendages which carry the pulmon- cally present, but is the ninth, owing to the prelamella:; I to VI, rudiments of the to term it, a proegenitcd ary six limbs of the prosoma; VIIPrG, the sence or to the excalation of a prsegenital somite. It prasgenital somite; VIII, the somite, hitherto unrecog- evanescent first mesosomatic somite or genital seems that confusion and trouble will be best avoided nized. In the case of somite; IX, the second mesosomatic by abstaining from the introduction of the non-evident or pectiniferous somite; X to Scorpio this segment is somite XIII, the four pulmoniferous somites; somites, the ocular and the prsegenital, into the numerical XIV, the first metasomatic somite. (After nomenclature of the component somites of the three great indicated in the embryo by Brauer, Zeitsch. wiss. Zool. vol. lix. 1895.) the presence of a pair of body regions. We shall, therefore, ignoring the ocular rudimentary appendages, carried by a well-marked somite. somite, speak of the first, second, third, fourth, fifth, and As in Limulus, so in Scorpio, this unexpected somite and 1 its appendages disappear in the course of development. See the article Akthropoda for the use of the term “prosthoIn fact, more or less complete “ excalation ” of the somite mere.”
