Three portions can be readily distinguished: fore-intestine with the
mouth, mid-intestine, as the seat of digestion and absorption, and
hind-intestine, or rectum, with the anal opening. The front and
hind-intestine are lined with infolded outer skin.
The nervous system consists of a supra-oesophageal ganglion with
four posterior nerve-cords--one dorsal, two lateral, and one (or
perhaps two) ventral. There were probably also remains of the old
plexus, but this is fast disappearing. The excretory system consists
of a pair of tubes discharging through the sides of the body-wall,
and having each a ciliated, funnel-shaped opening in the
perivisceral cavity. These have received the name of nephridia.
Through these also the eggs and spermatozoa are discharged. The
reproductive organs are modified patches of the peritoneum, or
lining of the perivisceral cavity.
The number of muscles or muscular layers has been reduced in this
animal. But such a reduction in the number of like parts in any
animal is a sign of progress. And the longitudinal muscles have
increased in size and strength, and the animal moves by writhing.
Such a worm has the general plan of the body of the higher forms
fairly well, though rudely, sketched. Many improvements will come,
and details be added. But the rudiments of the trunk of even our own
bodies are already visible.
Pages:
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87