Small organisms; especially Entomostraca and other small crustacea. When present one such will generally be found, tightly embraced by the endoderm.

Examine under a high power and look for evidences of digestion (intra-enteric) and assimilation.

b. Smaller organisms; especially Infusoria, Diatoms, and the like. Frequently to be met with in the central cavity; they may however be found, unchanged or in process of digestion (intra-cellular), within the individual cells of the endoderm.

8. The reproductive organs. (Cf. Sect. 2. a. e).

a. The testes; small conical colourless eminences below the point of attachment of the tentacles, more rarely irregularly scattered over the whole body.

Gently flatten out a testis in eosin or magenta by pressure on the coverslip, and examine with a high power. According to its state of maturity the following contents will be found in it—

a. A collection of the smaller ectoderm (interstitial) cells of variable shape.

B. The same, but having become ovoidal and hyaline. y. Cells otherwise like ß, but with a long filament proceeding from them.

8. Ripe spermatozoa; bodies consisting of a very small oval nucleated head to which a very delicate flagellum is attached, by the movements of which they swim about in the water. They may frequently be seen in motion within the unruptured testis.

b. The ovaries; one or more in number, larger than a, and situated near the base of the polype.

When young, each appears as a hill-shaped enlargement of the ectoderm; when ripe, it becomes rounded and very prominent.

c. Press out an ovary: according to its stage of development there will be found in it

a. Ectoderm cells with a marked preponderance of the smaller form (interstitial tissue).

B. Imbedded among a, one (ovicell) which has become larger and clearer than the rest, and possesses a distinct central germinal vesicle.

d. The ripe ovum. Conspicuous in H. viridis by its green colour. It consists of a great irregularly branched (amoeboid) mass of protoplasm (vitellus), in which is a clear space (germinal vesicle) containing one larger and a number of smaller germinal spots.

e.

Examine under a high power, and note—

a. The absence of a vitelline membrane.

B. The yolk granules; exceedingly large and modified to form the so-called 'pseudo-cells'. Each is rounded or oval, thickened on one side to form a plugshaped ingrowth and filled with a fluid contents. 7. The chloroplastids; present in H. viridis; identical with those found in the endoderm (Sect. 6. d. y).

The segmented ovum (oosperm): composed of a large number of small cells. Its thick horny capsule, rough on its external surface.

VII.

THE BELL-ANIMALCULE (Vorticella).

THE bell-animalcule is one of a very large group of animals called the Infusoria, on account of the fact that many members thereof make their appearance in infusions of certain animal and vegetable substances.

The higher multicellular animals begin their existence as simple nucleated cells, and the single nucleated cell which constitutes the whole animal in its primitive condition divides and subdivides until an aggregation of similar cells is formed. And it is by the differentiation and metamorphosis of these primitively similar histological elements that the organs and tissues of the body are built up. In the Infusoria, the protoplasmic mass which constitutes the germ does not undergo this process of preliminary subdivision, but such structure as the adult animal possesses is the result of the direct metamorphosis of parts of its protoplasmic substance. Hence, morphologically, the bodies of these animals are the equivalents of a single cell; while, physiologically, they may attain a considerable amount of complexity.

The Infusoria abound in fresh and salt waters, and many make their appearance, as before stated, in organic infusions, their germs either being contained in the substances infused, or being wafted through the air. Their diffusion is greatly facilitated by the fact that many of them retain their

vitality when dried, and reduced to the condition of an excessively light dust; while their rapid propagation is, in the main, due to their power of multiplying by division, with extraordinary rapidity, when duly supplied with nourishment. The majority are free and provided with numerous cilia by which they are incessantly and actively propelled through the medium in which they live; but some attach themselves to stones, plants, or even the bodies of other animals. A few are parasitic, and the bladder and intestines of the Frog are usually inhabited by several species of large size.

The Bell-animalcules are Infusoria which are fixed, usually by long stalks, to water plants, or, not unfrequently, to the limbs of aquatic Crustacea. They are barely visible to the unaided vision. The body has the shape of a wineglass with a very long and slender stem, provided with a flattened disc-like cover. What answers to the rim of the wine-glass is thickened, somewhat everted, and richly ciliated, and the edges of the disc are similarly thickened and ciliated. Between the thickened edge of the cover, or peristome, and the edge of the disc, is a groove, which, at one point, deepens and passes into a wide depression, the vestibulum. From this a narrow tube, the esophagus, leads into the central substance of the body, and terminates abruptly therein; and when fæcal matters are discharged, they make their way out by an aperture which is temporarily formed in the floor of this vestibule. The outermost layer of the substance of the body is denser and more transparent than the rest, forming a cuticula. Immediately beneath the cuticle it is tolerably firm and slightly granular, and this part is distinguished as the cortical layer or ectosarc; it passes into the central substance or endosare, which is still softer and more fluid.

In the undisturbed condition of the Bell-animalcule, the stem is completely straightened out; the peristome is everted, and the edges of the disc separated from the peristome; the vestibule gaping widely and the cilia working vigorously. But the least shock causes the disc to be retracted, and the edge of the peristome to be curved in and shut against it, so as to give the body a more globular form. At the same time, the stem is thrown into a spiral, and the body is thus drawn back towards the point of attachment. If the disturbing influence be continued, this state of retraction persists; but if it be withdrawn, the spirally coiled stem slowly straightens, the peristome expands, and the cilia resume their activity.

In the interior of the body, immediately below the disc, a space, occupied by a clear watery fluid, is seen to make its appearance at regular intervals-slowly enlarging until it attains its full size and then suddenly and rapidly disappearing by the approximation of its walls. This is the contractile vesicle or vacuole. It communicates with the exterior at the moment of contraction, and in all probability performs an excretory function. If the Bell-animalcule is well fed, one or more watery vesicles of a spheroidal form, each containing a certain portion of the ingested food, will be seen in the soft central mass of the body. And by mixing a small quantity of finely divided carmine or indigo with the water in which the Vorticella live, the manner in which these food-vesicles are formed may be observed. The coloured particles are driven into the vestibule by the action of the cilia of the peristome and the adjacent parts, and gradually accumulate at the inner end of the gullet. After a time the mass here heaped together projects into the central substance of the body, surrounded by an envelope of the accompanying water; and then suddenly breaks off,