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NEUROTRANSMITTERS IN NEMATODES 363
FIGURE 15.5 (See also Color Plate 10) Photograph of
Ascaris suum. This photograph shows Ascaris suum
within 24 hours of extraction from the host’s gut. The
pinkish color and the vitality of the nematodes indicate
that they are in good body condition. The photograph
also illustrates the large size of A. suum; adults
are 20–30 cm long and about 5 mm in diameter.
The muscle bag
The bag region of the cell contains the nucleus
and is rich in glycogen and mitochondria. It
therefore acts as the energy store for the muscle
cell. There is also an argument for a support
function for the muscle bag. In the intact
parasite, the bag bulges inwards towards the
gut; Harris and Crofton, in 1957, proposed that
the turgor of the muscle bags supports the
nematode, acting as an endoskeleton. Due to
the restricted number of cells in nematodes,
FIGURE 15.6 (A) Diagram of a cross-section through
Ascaris suum in a region 4 cm from the head. Note the
presence of the pseudocelom, dorsal and ventral
nerve cords, the lateral lines, and the separation of the
dorsal and ventral musculature. The hypodermis lies
under the cuticle between the muscle layer and cuticle.
(B) Diagram of the different regions of a muscle
cell from Ascaris.
after the maximum number of cell divisions
have taken place, growth continues by means
of cellular expansion. As a result, all the nematode
cells reach a relatively large size. The muscle
bags of Ascaris are 100–200 m in diameter
in fully grown worms, which facilitates insertion
of recording electrodes for electrophysiological
study.
The arm
The muscle arm is the connection from the
muscle cell to the nerve cord. The Ascaris neuromuscular
system differs markedly from that
found in vertebrates in that nematode muscle
cells have projections to the neurons, rather
than vice versa. Each muscle cell has one or
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS