Imagens das páginas
PDF
ePub

before it assumes its final characters. The eggs are laid in the water, and the young insect, when hatched, presents the appearances seen in fig. 17, B. It exhibits a head, thorax, with three pairs of legs, and abdomen; but it shows no traces of wings. It swims about actively in the water, and devours smaller insects by means of its powerful jaws. It breathes by means of a tuft of valvular appendages placed at the end of the abdomen, which can be opened so as to allow the water to gain access to the intestine, the sides of which are furnished with folds containing within them breathing-tubes. When the blood has been thus purified, the water is thrown out from the intestine, and the jet thus produced drives the animal in the opposite direction. After a while the insect passes into a second stage (fig. 17, C), in which it resembles the preceding in most respects, but has rudimentary wings (w) placed upon the back of the thorax. It is still active and voracious. Again after a while, the animal drags itself out of the water, and climbs upon some plant. Its skin then dries, and splits along the back, and the perfect insect, with its fully developed wings (fig. 17, A) is set free, and flies away to lead an active existence in the air. These remarkable changes constitute what is known as the “metamorphosis” of the insect (Greek, meta, indicating change, and morphe, shape).

Æshna grandis is the largest of British Dragon-flies, attaining a length of about two and a half inches. Its general colour is yellowish-brown, with two yellow lines on each side of the thorax, and the abdomen variegated with green or yellow spots. At all periods of its life, and especially in its winged state, it is a most active and voracious insect, living upon other insects; in its final stage, in spite of its ferocity and destructiveness, it is one of the most beautiful and graceful of insects, from its large and brilliant eyes, its lustrous wings, and the ease and power with which it performs the most rapid evolutions in the air.

RECAPITULATION OF ESSENTIAL CHARACTERS. — The body is composed of a succession of rings, and is divis

ible into three distinct regions—a head, thorax, and abdomen. The head carries a single pair of feelers (antennæ), the organs of the mouth, and the eyes. The thorax consists of three rings, and carries three pairs of legs. Generally, the last two segments of the thorax carry two pairs of wings. The adult insect breathes air directly, and the respiratory organs are in the form of branching breathing-tubes. The nervous system consists of a chain of little nervous masses placed along the lower surface of the body. The abdomen consists of distinct segments, which do not carry locomotive appendages. These characters distinguish the class of the Insecta as a whole.

CHAPTER XIII.

CLASS POLYZOA.

This class includes the curious Sea-mosses and Sea-mats, known technically as Polyzoa (Greek, polus, many; zoon, animal), because they consist of colonies or assemblages of little animals, associated so as to form compound growths—much in the way that a tree is composed of leaves and flowers supported upon a common trunk. As the type of this class we may take the broad-leaved Sea-mat or Hornwrack (Flustra foliacea), which is of common occurrence on the coasts of Britain.

This singular organism (fig. 19, a) is extraordinarily plant-like in form, and is generally regarded, when picked up on the shore, as being a pale-brown sea-weed. It forms a broad, and thin, leafy expansion, which is strongly rooted below by a common stem to a stone or some other foreign body, and which breaks up above into a number of flattened branches. Its consistence is horny, and its surface rough; and when it is examined with a magnifying glass, it is at once seen to be composed of an en

ormous number of little chambers or “cells," arranged in a single layer. These chambers (fig. 19, b) have horny

[graphic]

Fig. ra-Frustra poliacra, one of the Sea-mats. The plant-like colony,

natural size; A fragment of the colony maguitied, showing the little chambers or cells in which the separate animals forming the colony are

walls and are of an oval shape ; each having a little transverse opening or mouth near its broadest end, and having its upper margin provided with four conical spines Each cell contains a single animal, which leads an existence independent of the others; though the entire assemblage, made up of all the little animals contained in the innumerable cells has also a life of its own,

[ocr errors]
[ocr errors]

If we take a single cell of this compound growth, and examine the animal contained within it, we find that it has the following structure: The animal consists of a little membranous bag (fig. 20, en), which is closely applied to the horny wall of the cell (ec), and which is filled internally with a fluid, in which float the internal organs. From the mouth of the cell, the inner membranous sac can be partially thrust out; and here are situated the mouth and vent, close beside one another. The mouth (0) is surrounded by a circle of beautiful flexible processes (t),

Fig. 20.-Diagram of the animal contained in one of the cells of Flustra foliacea. ec Outer horny wall of the cell; en Inner

membranous wall enclosing the internal with innumerable vi

organ3; o Mouth, surrounded by tentacles organ3; Mouth, sy

(t); g Gullet: 8 Stomach ; i Intestine: prating nair-ulke nlia- v Vent; n Nervous system ; m Muscle ments. These hair-like by which the animal can pull itself into

its cell. processes are in constant movement, vibrating to and fro, and by their means currents are set up in the surrounding water, and particles of food are thus conducted to the mouth. When quiescent, or irritated, the animal can draw in the front portion of its body, with the tentacles, into the shelter of the horny cell.

The mouth conducts by a gullet (g) into a stomach (s); and this in turn opens into an intestine (*), which finally terminates in a distinct vent (v), placed on one

D

side of the mouth. To the gullet are attached certain muscular fibres (m), which are fixed below to the bottom of the cell ; and it is by the shortening of these that the animal pulls itself into the cell.

Between the mouth and vent is placed a little nervous mass, which constitutes the central portion of the nervous system (n). There is no heart, nor any blood-vessels ; nor are there any distinct breathing-organs. The fluid which fills the cell consists partly of water and partly of the products of digestion, and it is to be regarded as corresponding with the blood of higher animals. It is kept in movement by means of little vibrating hair-like processes which cover the membranous lining of the cell ; and it is exposed to the action of the oxygen contained in the water as it circulates through the crown of " tentacles," these latter organs being hollow.

The broad-leaved Sea-mat is of common occurrence in the seas of Britain, in a few fathoms of water. It attains a height of about four inches, and has a wood-brown colour. When fresh, it is stated to possess a peculiar odour (not always present), which has been variously compared to the scent of oranges, violets, or a combination of roses and geraniums, but which others consider strong and disagreeable (Johnston).

RECAPITULATION OF ESSENTIAL CHARACTERS.—Animal compound, consisting of numerous, nearly independent beings, each of which is enclosed in a separate chamber or cell. (This last character is not absolutely universal.) Each member of the compound growth has a mouth surrounded by tubular tentacles, à complete alimentary canal opening by a distinct vent, and a nervous system consisting of a little nerve-centre placed on one side of the mouth. There is no heart, nor are there definite breathing-organs. Almost universally the colony is attached to some foreign object. These characters distinguish the Polyzoa as a whole.

« AnteriorContinuar »