Page:Encyclopædia Britannica, Ninth Edition, v. 22.djvu/437

 SPONGES 413 of the Ascetta, as of all sponges, is most obviously mani- fested, as Grant (5) first observed, by a rapid outflow of water from the oscule and a gentle instreaming through the pores, a movement brought about by the energetic action of the flagella of the endodermic cells. The in- streaming currents bear with them into the cavity of the sac (paragastric cavity) both protoplasmic particles (such as Infusoria, diatoms, and other small organisms) and dissolved oxygen, which are ingested by the flagellated cells of the en- dodenn. The presence of one or more contractile vacuoles in these cells suggests that they extricate water, urea, and car- bonic acid. The insoluble re- sidue of the introduced food, together with the fluid excreta, is carried out through the os- cule by the excurrent water. Xew individuals are produced from the union of ova and spermatozoa, which develop from wandering amoeboid cells in the mesoderm. The walls of Ascetta are strengthened by calcareous scleres, more especi- rrr - r ri ' ' > T-T-T-T-^ r ally designated as SpiculeS, FIG. 2. Homoderma sycandra, Lfd. wlnVli IIQVP tTio fnrm nf tri One half cut awa y b y a vertical unicn na tlie median section. After V. Lemlen- radiate needles. If we make feid(x about 6). abstraction of these we obtain an ideal sponge, which Haeckel has called Olynthus (6), and which may be re- Canal System. We shall now trace the several modifi- cations which the Olynthus has undergone as expressed in the different types of canal system. The simple paragaster of Ascetta may become compli- Ascon cated in a variety of ways, such as by the budding off *}"]' from a parent form of stolon-like extensions, which then give rise to fresh individuals, or by the branching of the Ascon sac and the subsequent anastomosis of the branches ; but in no case, so long as the sponge remains within the Ascon type, does the endoderm become differentiated into different histological elements. The most interesting modification of the Ascon form occurs in Homoderma sy- candra (i2 in which from the walls of a simple Ascon csecal processes grow out radiately in close regular whorls, each process reproducing the structure of the parent sponge (figs. 2, 3). From this it is but a short step to the important departure which gives rise to the Sycons. In the simplest examples of this type the characters of Sycon Homoderma sycandra are reproduced, with the important tjT e - exception that the endoderm lining the paragastric cavity of the original Ascon form loses its primitive character, FIG. 3. Homoderma sycandra, Lfd. Transverse section, showing radial tubes opening into central paragastric cavity. After V. Lendenfeld (x about 1'J). garded as the ancestral form from which all other sponges have been derived. To give greater exactness to our ab- straction we should perhaps stipulate for the Olyntlius a somewhat thicker mesoderm and more spherical form than a decalcified Ascon presents. FIG. 4. Heteropegma, n odus-(to/-di i, Pol. Part of a transverse section. The straight lines indicate spicules ; the poriferous surface is uppermost ; the branching radial tubes are rendered dark by numerous small circles representing ehoanocytes. After Polejaeff, " Cliallenger" Report (x50). and from a layer of flagellated cells becomes converted into a pavement epithelium, not in any distinguishable feature different from that of the ectoderm. The flagellated cells are thus restricted to the caecal outgrowths or radial tubes. Concurrently with this differentiation of the endoderm a more abun- dant development of mesoderm occurs. In some Sycons (Sycaltis, Hk.) the radial tubes remain separate and free ; in others they lie close together and are united by trabecuke, or by a trabecular network, consisting of mesodermic strands sur- rounded by ectoderm (fig. 4). The spaces between the contiguous radial tubes thus become converted into narrow canals, through which water passes from the exterior to enter the pores in the walls of the radial tubes. These canals are the " inter- canals " of Haeckel, now generally known by their older name of incurrent canals. The openings of the incurrent canals to the exterior are called pores, a term which we have also applied to the openings which lead directly into the radial tubes or paragastric cavity ; to avoid ambiguity we shall for the future distinguish the latter kind of open- ing as a prosopyle. The term "pore" will then be restricted to the sense in which it was originally used by Grant. The mouth by which a radial tube opens into the paragaster is known as a gastric ostium. In the higher formsof Sycons the radial tubes nolonger arise as simple out- growths of the whole sponge-wall, but rather as outgrowths