Write a brief overview of the endocrine system of

(a)  cnidarians’ (b) platyhelminths (c) nemerteans (d) nematodes and (e) annelids.

(a) Cnidarians

The nerve cells of Hydra contain a growth promoting hormone that stimulates budding, regeneration, and growth. For example, when the hormone is present in the medium in which fragments of Hydra are incubated, “head” regeneration is accelerated. This so called “head activator” also stimulates mitosis in Hydra.

(a)  Platyhelminths

Zoologists identified neurosecretory cells in various flatworms over 30 years ago. These cells are in the cerebral ganglion and along major nerve cords. The neuropeptides that the cells produce function in regeneration, asexual reproduction, and gonad maturation. For example, neurosecretory cells in the scolex of some tapeworms control shedding of the proglottids or the initiation of strobilizabon.

(b)  Nemerteans

Nemerteans have more cephalization than platyhelminths and a large brain, composed of a dorsal and ventral pair of ganglia connected by a nerve ring. The neuropeptide that these ganglia produce appears to control gonadal development and to regulate water balance.

(d)     Nematodes:

They have neurosecertory cells associated with the central nervous system. The neuropeptide that this nervous tissue produces apparently controls ecdysis of the old cuticle. The neuropeptide is released after a new cuticle is produced and stimulates the excretory gland to secrete an enzyme (leucine aminopeptidase) into the space between the old and new cuticle. The accumulation of fluid in this space causes the old cuticle to split and be shed.

(d)     Annelids:

The various endocrine systems of annelids are generally involved with morphogenesis,  development, growth, regeneration, and gonadal maturation. For example, in polychactes, juvenile hormone inhibits the gonads and stiumlates growth and regeneration. Another hormone, gonadotropin, stimulates the development of eggs. In leaches, a neuropeptide stimulates gamete development and triggers color changes. Osmoregulatory hormones have been reported in oligochactes, and ahyperglycemic hormone that maintains a high concentration of blood glucose has been reported for the oligochacte, Lumbricue.




Figure 3.9

How do hormones function in the common land snail Helix, and in cephalopods?

In certain gastropods, such as the common land snail Helix, a specific hori stimulates spermatogenesis; another hormone, termed egg-laying hormone, stimulates egg development; and hormones from the ovary and testis stimulake accessory sex organs. In all snails, a growth hormone controls shell growth.

In cephalopods, such as the octopus, and squid, the optic gland in the eye stalk produces one or more hormones that stimulate egg development, proliferation ID: spermatogonia, and the development of secondary sexual characteristics.

What is the function of neuropeptide in sea star?

 The radial nerves of sea stars contain a neuropeptide called gonad — stimulating substance. When this neuropeptide is injected into a mature sea star, it induces immediate shedding of the gametes, spawning behaviour, and meiosis in the ocytes. The neuropeptide also causes the release of a hormone called maturation — inducing substance, which has various effects on the reproductive system.

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