IRRITABILITY OF SIMPLE ANIMALS

Unicellular Animals.

Amoeba and Paramecium show no trace of structures for the conveyance of stimuli from one part to another, yet in some way not at present understood each organism responds either as a whole or at least with some part of itself, making a useful response to useful stimuli and a defensive response to harmful ones.

Multicellular Animals.

Hydra

We find in Hydra a very simple type of nervous system. Nerve cells are situated on both sides of the mesoglcfia, their Nerve Net of Hydra and its Connections. processes connecting with the sense cells, muscle tail cells and with other nerve cells, so that altogether they form a nervous network throughout the animal. The three principal stimuli are touch, light and the presence of food, and the responses are tabulated below.

We see from this table that the nerve network is involved in cases marked *. Since there are no special parts to this network, the impulses spread to the surrounding cells, the extent of the response showing the extent to which they spread. The other cases are those where the cells responding are also the receptors, being similar in this respect to unicellular organisms. Despite the fact that the nerve network is generalized, I.e. there are no nerve centres, the animal can carry out some quite specialized movements, e.g. looping and somersaulting.

Earthworm and Cockroach

All the more complex invertebrate animals have a nervous system consisting of nerves coming from a central nervous system which lies below the gut and is double, ventral and solid, unlike that of vertebrates, which is single, dorsal and hollow, and possesses paired ganglia at roughly regular intervals corresponding to the segments. At its anterior end, the two strands of nervous tissue separate to* encircle the gut, joining above it in two prominent ganglia commonly called the ’brain.

The whole system is made up of neurons arranged in reflex arcs very much as in the case of the spinal cord and its nerves in a vertebrate, but there is no structure corresponding to the brain of vertebrates.

We do not know to what extent the animal is conscious of its activities, for most of them are of the instinctive reflex type, although quite complex co-ordinated reactions are carried out. These are well exemplified in the case of hive bees, where each worker has a series of duties to carry out in a definite order, from the time she begins as a ’nurse bee ’until she dies. None of these animals is taught how to do any of these things. It may be argued that they learn to discriminate their keeper from other folks, but even such recognition may be of the conditioned reflex type. Intelligence as we know it docs not appear to be exhibited, but some arthropods may learn, e.g. the performing fleas in the old-fashioned circus.

A marked difference between the muscular responses exhibited by the earthworm and the cockroach is due to the fact that the muscles of the earthworm are of the smooth type, which carry out steady, slow, but not violent contractions, whereas those of the cockroach are striped and carry out intermittent, rapid and quite powerful contractions. The wing muscles are amongst the most efficient known, and the nervous control of the two sets involved in raising and lowering the wings must be of a high order. The rate of vibration of the wings of a fly is approximately 330 times per sec., as determined by the note they make and by experiments where the insect ’s wings are made to trace a line on a smoked revolving drum.

The sense organs of these types have already been dealt with in Chaps. Ill and IV.

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