The internal organs.—Figs. 163 and 223, which represent a general dissection of a frog and of a cockroach respectively, exhibit in the most striking manner the essential differences in the structure of vertebrates and arthropods. The fact that the skeleton of the frog is wholly internal and that of the cockroach wholly external, has [Pg 356] already been mentioned. It will be seen, further, that while in the frog the central nervous system (the brain and spinal cord) lies entirely dorsal to the digestive canal, in the cockroach the great nerve chain (Fig. 223, n, n,) is mainly ventral—the only dorsal part of the central nervous system being the so called brain (brn.), which is connected with the ventral chain by a ring surrounding the gullet (gul.). The heart of the frog is ventral to the digestive canal; that of the cockroach is the most dorsally placed organ of the body. Fig. 223.—Cockroach; general dissection of female from the left side. abd. 1, first, and abd. 5, fifth abdominal segments; brn., “brain”; cer., cercus; c.gl., glands which form the egg-case; cr., crop; f., feeler; giz., gizzard; gul., gullet; int., intestine; lab. pa., labial palp; mx. pa., maxillary palp; n, n, nerve chain; r. ov., right, and l. ov., left ovary; sal. du., salivary duct; sal. gl., salivary gland; spir., spiracles; st. 7, brood-chamber; th. 1, first, th. 2, second, and th. 3, third thoracic segments. (× 2.) Habits and life-history.—Cockroaches infest kitchens and pantries; they are of social habits, and hide together in crevices during the day, but come forth at night to feed. They are not at all [Pg 357] fastidious as to diet, but are especially fond of starchy foods, which they are able to digest (p. 234) by means of the fluid formed in their large salivary glands (Fig. 223, sal. gl.). The eggs are laid sixteen at a time in a little oblong case, which the female carries about in a boat-shaped receptacle (st. 7, Fig. 223) at the end of her abdomen, until she finds a suitable place in which to deposit it. When the little cockroaches hatch, they are quite white, but except for the absence of wings they closely resemble the parents in form, and run about and feed freely from the first. The chitinous exoskeleton is shed from time to time as the animal increases in size, a new coat, at first soft and wrinkled, but rapidly stretching and hardening, having previously formed beneath the old one. Shortly before the last moult, the wing-covers and wings begin to grow out from the angles of the second and third thoracic segments. In this manner the young animal gradually takes on the form and dimensions of the adult.