The tergal regions of the mesothorax and of the metathorax are hidden under the pronotum and the elytra when the latter are closed, except that the mesothoracic scutellum is of ten visible - a small triangular or semicircular plate between the bases of the elytra (fig.
Tergal ful muscles arising from the thoracic walls, and inserted into the proximal ends of the thighs, N.
The dorsal vessel is an elongate tube, Oblique tergal whose abdominal portion is usually c ha m b e r e d, forming a contractile heartfi Io the (g io).) At th ' constrictions between the pericardium l?': ?:' chambers are paired slits, through which the blood passes from the surround Tergo-sternal .
A tendency is exhibited to the formation of a metasomatic as well as a prosomatic carapace by fusion of the tergal surfaces of the somites.
A telsonic tergal shield of greater or less size is always present, which may be imperfectly divided into well-marked but immovable tergites indicating incompletely differentiated somites.
This telson may enlarge, it may possibly even become internally and sternally developed as partially separate somites, and the tergum may remain without trace of somite formation, or, as appears to be the case in Limulus, the telson gives rise to a few well-marked somites (mesosoma and two others) and then enlarges without further trace of segmentation, whilst the chitinous integument which develops in increasing thickness on the terga as growth advances welds together the unsegmented telson and the somites in front of it, which were previ ously marked by separate tergal thickenings.
On the other hand, an unusually large tergal plate, whether terminal or in the series, is not always due to fusion of the dorsal plates of once-separate somites, but is of ten a case of growth and enlargement of a single somite without formation of any trace of a new somite.
It is represented by a full-sized tergal plate in the Pseudo-scorpiones.
A prae-genital somite is often present either in a reduced condition forming a waist (Pedipalpi, Araneae, Palpigradi) or as a full-sized tergal plate (Pseudoscorpiones); in some it is entirely atrophied (Solifugae, Holosomata, and Rhynchostomi).
P, The tergal stigmata of the tergo-sternal muscles.
There is also a pair of narrow tergal sclerites interposed between the anterior and posterior shields.
- Opisthosoma dis tinctly segmented, furnished with tergal plates, as in the Ambly pygi; the ventral surface of the 1st and 2nd somites with large sternal plates, covering the genital aperture and the two pairs of FIG.
Opisthosoma consisting of only ten somites, which have no tergal and sternal elements, the prae-genital somite contracted to form a " waist," as in the Pedipalpi; the last three narrowed to form a A B prae-1 2345 6789 io I I111I IV V VI gen Opisttaosoma Prosoma FIG.
C, Lateral view, I to VI, proprosternites; c, mesosternite; and d, metasternite of the somatic appendages; a,b,c, prosoma; f, ventral surface the three tergal plates of the of the prae-genital somite; prosoma; prae-gen, the prae g, sternite of the genital genital somite; 1 to 10, the somite (first opisthosomatic ten somites of the opisthosoma.
The prae-genital somite wide, not constricted, with large tergal plate, but with its sternal plate small or inconspicuous.
- Dorsal area of prosoma furnished with two shields, a larger behind representing, probably, the tergal elements of the somites, and a smaller in front, which is freely articulated to the former and folds over the a b C an FIG.
Opisthosoma confluent throughout its breadth with the prosoma, with the dorsal plate of which its anterior tergal plates are more or less fused; at most ten opisthosomatic somites traceable; the generative aperture thrust far forwards between the basal segments of the 6th appendages.
A, Lateral view with appendages III to VI removed; 1, plate covering the whole dorsal area, representing the fused tergal sclerites of the prosoma and opisthosoma; 2, similarly-formed ventral plate; 3, tracheal stigma.
This head-shield is succeeded by a varying number of free segments, each of which consists of a medium convex tergal piece and a pair of arched lateral plates, the pleura, of which there is one on each side.
The tergal and pleural elements of the pygidium are generally well marked.
They are also well marked on the cephalic shield, the tergal elements being represented by a median axial elevated area showing indistinct signs of segmentation, and a lateral unsegmented plate, the gena, which carries the eyes.
In the majority of Trilobites this groove passes backwards from the anterior or anterolateral edge of this plate to its posterior or postero-lateral border, dividing it into an inner portion continuous with the flabellum and fused tergal regions, and an outer portion bearing the eye.