abdominal artery—vessels which run in close connection with the nerve cord
and which supply the limbs and other organs of the body.
Examination of a Larval Form (Phyllosoma).
Examine under the microscope, or with a lens, a larval form (Phyllosoma) of


the Crawfish. Note and draw :—-
(a) The large head region, distinct from the thoracic region, and the very


small abdominal region.
112 THE FRESH-WATER CRAYFISH—THE CAPE CRAWFISH


(b) The two well-developed eyes, and a median eye spot.
(c) The antennules.
(d) The antenne, with an exopodite and an endopodite branch.
(e) The third maxilliped.
(f) The first and second walking legs, with well-developed feather-like
exopodites.
(g) The long third pair of walking legs, with the rudiment of the exopodite
on the first segment of the limb. The fourth and fifth walking legs
and the appendages of the abdomen have not yet appeared.
-- Endopodite
Antennule


Antenna \ \ Z | / _.Exopodite


Eye-.
Exopodite


[ ---Endopodite


Fra. 34.—Phyllosoma larva of the Crawfish. Enlarged.
THE SNAIL (Helix aspersa)


External Features.
1. Make a drawing of the Snail, viewed from the right side, to show :


(a) The univalve spirally coiled shell (a right-handed conical spiral) within
which is the twisted visceral hump containing the chief internal organs.
Round the mouth of the shell is the collar, the thickened edge of the
mantle.
. (0) The head, with two pairs of tentacles and the mouth. The mouth is
situated ventrally at the anterior end of the head. The tentacles are
retractile ; the upper and larger pair have eyes at their tips, the lower
and smaller pair arise in front of the larger ones.
(c) The large muscular foot, constituting the organ of locomotion, on which
the animal glides. It is the flattened ventral part of the body-wall.
‘ie.
