. A manual of zoology. Zoology. II. ACERATA: ARACHNIDA 389 Order II. Eurypterida. Extinct Silurian and Devonian forms with small cephalothorax and large twelve-jointed abdomen; intermediate between xiphosures and scorpions. Eurypterus; Ptcrygotus, some species seven feet long. Sill) Class II. Araclmida. Under this name are included a number of orders of greater or less extent which can be arranged around the spiders, or Aranea, as a centre. There is considerable modification of form, and the follomng account applies only to the more typical groups. In these the cephalothorax and abdomen are se
. A manual of zoology. Zoology. II. ACERATA: ARACHNIDA 389 Order II. Eurypterida. Extinct Silurian and Devonian forms with small cephalothorax and large twelve-jointed abdomen; intermediate between xiphosures and scorpions. Eurypterus; Ptcrygotus, some species seven feet long. Sill) Class II. Araclmida. Under this name are included a number of orders of greater or less extent which can be arranged around the spiders, or Aranea, as a centre. There is considerable modification of form, and the follomng account applies only to the more typical groups. In these the cephalothorax and abdomen are separated by a distinct hne, and since the abdominal ap- pendages almost entirely disappear in the adult, the number of abdomJnal somites can only be ascertained where their boundaries are e\-ident. The number varies between six in the phalangids and thirteen in the scorpions. The cephalothorax is, except in the Solpugidffi, a single piece which bears six pairs of appendages; the four posterior pairs, each typically seven jointed, are locomotor, so that eight legs are as characteristic for an arachnid as ten for a decapod or six for a hexapod. The first pair of appendages, the chelicerce (fig. 419), are preoral, the second, or pedipalpi, beside the mouth. The chelicerffi are short and consist of two or three joints, the terminal joint either folding back upon the other or, pincer-like, meeting an opposable thumb. In the spiders the last joint or claw is forced fig. 419.—Mouth parts of into the prey, introducing poison from a sac Epeira. i, chehcerfe; 2, pedi- ... rr^i 1- 1 • palpi;/>, palpus;/, basal plate. in the basal jomt. The pedjpalpi are elon- gate, leg-like, their basal joints often forming a lip, the other joints form- ing the palpus, which may end with a claw or a pincer. The question has often been discussed as to whether the chelicerae are the homologues of the antennas of other arthropods. The embryological evidence is in favor of their equivalence to the secon
Size: 1533px × 1631px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
