1. Fungia agariciformis; 2. Alcyonium, Cydonium Mulleri; 3. Cydonium, polyps protruding and tentacles expanded, others closed; 4. A stock viewed from above; 5. Madrepore abrotanoide; 6. Madrepore, slightly magnified, showing oral opening; 7. Corallidæ; 8. Coral, polyps protruding from cells; 9. Gorgonia nobilis, with polyps expanded; 10. Tubipora musica; 11. Tubes of same, with polyps expanded, one cut longitudinally to show internal structure; 12. Sertularia, polyps protruded, and withdrawn into their polypidoms.
Fig. 359.
1. Coryne stauridia; 2. A tentacle detached and magnified 200 diameters.
There are many beautifully formed freshwater polyps deserving of more than a passing notice, as the slender Coryne (Coryne stauridia), found adhering to the footstalk of a Rhodymenia ([Fig. 359]), about which it creeps in the form of a white thread. On placing both under the microscope, the thread-like body of the little animal appears cylindrical and tubular, perfectly transparent, and permeated by a central core, apparently cellular in texture, hollow, and within which a rather slow circulation of globules is perceived. The parent Coryne sends off numerous branches, the terminal head of which is oblong, cylindrical, and at the extreme end there are arranged four tentacles, long and slender, each being furnished with a nodular head. A magnified view of one detached is shown erect ([Fig. 359], No. 2). This polyp is much infested by parasites, vorticella growing on it in immense numbers, forming aggregated clusters here and there, individuals of the parasitic colony adhering to each other, and projecting outwards in every direction.
Alcyonella, another fresh-water polyp, is found in the autumn of the year in all the London Docks adhering to pieces of floating timber. A. stagnorum partakes of the character of a sponge rather than that of a polyp. It is usually found in gelatinous colonies, and when stood aside for a short time these put forth a number of ciliated tentacles (shown in [Fig. 360], magnified 100 diameters).
The ova contained within the sac, and viewed by transmitted light, appear as opaque spheres surrounded by a thin transparent margin; these increase in thickness as the ova is developed, and such of the ova as lie in contact seem to unite and form a statoblast. A rapid current in the water around each animal, drawing with it loose particles and floating animalcules, is seen moving with some velocity as in other ciliated bodies; and a zone of very minute vibrating cilia surrounds the transparent margin of each tentacle.
Fig. 360.—Alcyonella fluviatella.
Dr. Percival Wright discovered on the western coast of Ireland a new genus of Alcyonidæ, which he named after the well-known naturalist Harte, Hartea elegans ([Plate IV]., No. 86). This polyp is solitary, the body cylindrical, and fixed by its base to the rock; it has eight ciliated tentacles, which are knobbed at their base and most freely displayed. It is a very beautiful polyzoon of a clear white colour, and when fully expanded stands three-quarters of an inch high.