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Show 16 ON QJ_,ASSIFICATION. Now that we know the whole cycle of tho life of the sponges, and the characters which may be demonstrated to be common to the whole of this important and remarkable class, I do not think any one who is acquainted with the .organization or ~he functions of plants, will be inclined to a~~I~ that the Spongttda have the slightest real affinity with any drvlsion of the vegetable kingdom. . ... The next group to be considered IS the diVISion of the INFU-SORIA; and here, again, within the last few years, prodigious strides have been made in our knowledge of the subject. Although the Infusoria have been favourite studies for many years, still it is only quite recently that the cycle of life of these animals has been made almost completely known, and that we have become acquainted with the true sexual process as it occurs in thmn. Fig. 4. Fig. 4.-Parammciurn bursa!'ia (after Stein): A, The animal viewed from the dorsal side; a, cortical l:-1yer of the body; b, "nucleus;" c, contractile chamber; d d', matters taken in as food; e, chlorophyll granules. B, The animal viewed from the ventral side; a, depref'sion leading to b, mouth ; c, gullet; d, "nucleus;'' d', "nucleolus;" e, central sarcodc. In both these figures the arrows indicate the direction of the circulation of the sarcode. C, Pararnceciurn dividing transversely; a a', contractile spaces; b b', "nucleus" dividing; c c', "nucleoli." The different species of the genus Paramcecium are very common among the microscopic inhabitants of our fresh waters, swimming about by means of the vibratile cilia with which the whole surface of their bodies is covered; and the structure which essentially characterises these animals is probably that which is common to the whole of the Injusor~:a, so that an account of the GREGAIUNIDA, RIIIZO!JODA, SPONGIDA, AND INFUSORIA. 17 leading strnrtural features of Paramcecium is, in effect, a definition of tho e of the group. I1nagino a delicate, slipper-shaped body inclosed within a structureless 1nmnbrane, or cuticula, which is formed as an excretion upon its outer surface. At one point (Fig. 4, B a) tho body exhibits a slight depression, leading into a sort of little funnel (b c) coated by a continuation of the , ·an1o cuticular investment, which stops short at the botton1 of tho funnel. ~:rhe whole of the bag formed by the cuticula is lined by a soft layer of gelatinous Inatter, or '~ sarcode," which is called the "cortical" layer (Fig. 4, A a) ; while inside that, and passing into it quite gradually, there being no sharp line of demarcation between the two, is a semi-fluid substance, which occupies the whole of the central region of tho body. Neither in tho cuticle, the cortical layer, nor the central substance, has any anatomist yet discovered a differentiation into cellular layers, nor any trace of that histological composition which we meet with in the tissues of the higher animals; so that here is another case of complex vital phenomena proceeding from a substance which, in a histological sense, is structureless. At two points of the body (Fig. 4, A c c) the substance of the cortical layer exhibits a remarkable power of contraction and dilatation. If you watch one of those points, the sarcode suddenly seen1s to open like a window and, for a while, a clear space is visible, which then, quite suddenly, shuts again. After a little time the same diastole and systole are repeated. As the systole takes place, it is possjble, occasionally, to discern certain radiating canals, which extend from the cavities into the sp.rrounding sarcode, and disappear again before diastole occurs. There is no doubt that the clear space is a chamber filled with fluid in the cortical layer, and since good observers maintain that there is an aperture of cmnmunication, through the cuticula, between the 'contractile chamber ' and the exterior, this fluid can be little more than water. Perhaps the whole should be regarded as a respiratory or secretory 1nechanism: in one shape or another, it is eminently characteristic of the Infusoria. Besides this singular apparatus, there li ,s on1boddod in another part of tho cortical layer a solid mass, c |