laterally to large gastric glands, which are more than a single pair
in number (two to six pairs), and may assume the form of simple caeca.
The mouth is minute and the pharynx is always suctorial, never
gizzard-like. The gonadial tubes (gonocoels or gonadial coelom) are
originally reticular and paired, though they may be reduced to a
simpler condition. They open on the first somite of the mesosoma. In
the numerous degenerate forms simplification occurs by obliteration of
the demarcations of somites and the fusion of body-regions, together
with a gradual suppression of the lamelliferous respiratory organs and
the substitution for them of tracheae, which, in their turn, in the
smaller and most reduced members of the group, may also disappear.

The Eu-arachnida are divided into two grades with reference to the
condition of the respiratory organs as adapted to aquatic or
terrestrial life.

Grade a (of the Eu-arachnida). DELOBRANCHIA (Hydropheustea).

Mesosomatic segments furnished with large plate-like appendages, the
1st pair acting as the genital operculum, the remaining pairs being
provided with branchial lamellae fitted for breathing oxygen dissolved
in water. The prae-genital somite partially or wholly obliterated in
the adult. The mouth lying far back, so that the basal segments of all
the prosomatic appendages, excepting those of the 1st pair, are
capable of acting as masticatory organs. Lateral eyes consisting of a
densely packed group of eye-units ("compound" eyes).

Order 1. Xiphosura.--The prae-genital somite fuses in the embryo with
the prosoma and disappears (see fig. 19). Not free-swimming, none of
the prosomatic appendages modified to act as paddles; segments of the
mesosoma and metasoma (=opisthosoma) not more than ten in number,
distinct or coalesced.

Family--Limulidae (_Limulus_).
" *Belinuridae (_Belinurus_, _Aglaspis_, _Prestwichia_).
" *Hemiaspidae (_Hemiaspis_, _Bunodes_).

[Illustration: FIG. 44.--Dorsal view of _Limulus polyphemus_, Latr.

(From Parker and Haswell, _Text book of Zoology_ after Leuckart.)]

_Remarks_.--The Xiphosura are marine in habit, frequenting the shore.
They are represented at the present day by the single genus _Limulus_
(figs. 44 and 45; also figs. 7, 9, 11, to 15 and 20), often termed the
king-crab, which occurs on the American coast of the Atlantic Ocean,
but not on its ea