(43). In one species of Amteba (A. verrucosa, Ehr.) Mr. Carter* has witnessed ovular development, the Amoeba perishing as the ovules are perfected, and ending in becoming a mere ovisac. When first formed, the ovules, which are spherical, consist of a hyaline capsule enclosing a sphere of glairy, refractive fluid; but as they begin to increase, this glairy matter becomes transformed into a granuliferous mucus which is spread over the inner surface of the capsule; and finally the granules present motion - whether of themselves or by the aid of the mucus in which they are imbedded is uncertain. The history of their further development has not yet been made out; but Mr. Carter thinks that the next stage of their growth consists in the whole ovule becoming polymorphic.

(44). Sponges

However dissimilar apparently, both in form and structure, from the simple organisms described above, it is in their immediate vicinity that we must place the extensive group of Sponges, which has until recently held a very dubious position upon the confines of the animal and vegetable kingdoms.

(45). The common sponge of commerce is, as every one knows,made up of horny, elastic fibres of great delicacy, united with each other in every possible direction, so as to form innumerable canals which traverse its substance (fig. 9, c.) To this structure the sponge owes its useful properties, the resiliency of the fibres composing it making them, after compression, return to their former state, leaving the interstitial canals open, to suck up surrounding fluids by capillary attraction.

(46). The dried sponge is, however, only the skeleton of the fabric. In its original state, before it was withdrawn from its native element, every filament of its substance was coated over with a thin film of glairy, semifluid matter that constitutes the living part of the sponge, secreting, as it extends itself, the horny fibres which are imbedded in it.

Spicula and horny skeleton of various Sponges.

Fig. 9. Spicula and horny skeleton of various Sponges.

* Ann. & Mag. Nat. Hist. 2nd ser. vol. xx. p. 37.

(47). Many species, although exhibiting the same porons structure, have none of the elasticity of the officinal sponge, a circumstance to be attributed to the difference observable in the composition of their skeleton or ramified framework. In such, the living investment forms within its substance not only tenacious bands of animal matter, but great quantities of crystallized spicula, sometimes of a calcareous, at others of a silicious nature, united together by the tenacity of the fibres with which they are surrounded. On destroying the softer portions of these skeletons either by the aid of a blowpipe or by the caustic acids or alkalies, the spicula remain, and may readily be examined under a microscope: they are then seen to have determinate forms, generally in relation with the natural crystals of the earths of which they consist; and as the shape of the spicula is found to be similar in all sponges of the same species, and not unfrequently peculiar to each, these minute particles become of use in the identification of these bodies.

(48). Crystallized spicula of this description form a feature in the structure of the sponge which is common to that of many vegetables, resembling the formations called raphides by botanical writers. Some of the principal forms they exhibits are depicted in fig. 9, a, b, d, e, f, g, which likewise will give the reader a general idea of the appearance of the silicious and calcareous sponges after the destruction of their soft parts has been effected by the means above indicated. The figures d, e, f, and g likewise represent detached spicula of different shapes highly magnified. The most convenient method of seeing them is, simply to scrape off a few particles from the incinerated sponge upon a piece of glass, which, when placed under the microscope, may be examined with ordinary powers.

(49). On placing a living sponge of small size in a watch-glass or small glass trough filled with sea-water, and watching it attentively, something like a vital action becomes apparent 1. The entire surface is seen to be perforated by innumerable pores and apertures, some exceedingly minute, opening on every part of its periphery; others of larger dimensions, placed at intervals, and generally elevated upon prominent portions of the sponge. Through the smaller orifices the surrounding water is continually sucked as it were into the interior of the spongy mass, and it as constantly flows out in continuous streams through the larger openings. The annexed diagram, fig. 10, A, will give the reader an idea of the most usual direction of the streams. The entering fluid rushes in at the countless pores distributed over the general surface of the sponge, but in its progress through the canals in the interior becomes directed into more capacious channels, communicating with the prominent larger orifices, through which it is ultimately ejected in equable and ceaseless currents. Organized particles, such as necessarily abound in the water of the ocean, are thus introduced into the sponge on all sides, and are probably employed as nutriment, whilst the superfluous or effete matter is continually cast out with the issuing streams as they rush through the fecal orifices. The growth of the sponge is thus provided for; the living gelatinous portion continually accumulates and, as it spreads in every direction, secretes and deposits, in the form peculiar to its species, the fibrous material and earthy spicula constituting the skeleton.

* Savigny (Jules Cesar), Zoologie d'Egypte: gr. fol. Paris, 1809. 1 Dr. Grant, in the New Edinburgh Philosophical Journal, 1827.

(50). It is by no means easy to explain the cause of the perpetual flow of water through the substance of the sponge in currents so powerful and so constant. In the various species of Grantia, however, Mr. Bowerbank and Dr. Dobie have succeeded in detecting the presence of cilia. These sponges have a very simple structure, each being a sort of bag, whose walls are so thin that no system of canals is required, the water absorbed by the outer surface passing directly towards the inner, and being expelled from the mouth of the bag. The cilia may be plainly seen with a 1/8th inch objective on the cells of the gelatinous substance scraped from the interior of the bag, or they may be observed in situ by making very thin sections of the substance of the sponge.