Licence: Public Domain Mark
Credit: A history of the British zoophytes / by George Johnston. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
54/460 page 34
![for the present, to consider all the ascidian as compound poly- pes.* '^I'here is nevertheless a remarkable difference between them and the hydraform tribes in their mode of composition. In the latter the polypes are simply developements of the com- mon central fleshy mass, identical with it in structure and tex- ture ; in the former each individual is a distinct organism, and the medium which binds them together, whether vascular or li- gamentous, has its own peculiar character. The one we may compare to a chain of which all the links are welded,—the other to a necklace where the beads are strung together by a percur- rent thread. To express this distinction we shall call the hydra- form compound polypes, and the ascidian aggregated polypes. The body of the ascidian polypes is lengthened, somewhat cylindrical or at times bulged at the base, and when at rest lies, in the form of a syphon, doubled up upon it- self in the cell, (Fig. 2, f) to which it is con- nected by a tendon at the bottom, and by the duplicature of a thin membrane round the aperture, so that it is impossible it should ever voluntarily leave the cell to swim at large, as Baster and others have maintained. The head or upper end is surrounded by a single row of tentacula, (Fig. 3, a,) which are solid, filiform and non-contractile, for the animal can only shorten them, excepting to a slight extent, by rolling them up in a spiral manner: they are apparently smooth, but with a high mag- nifier it is ascertained that they are clothed with numerous fine cilia, ^ which are in ceaseless motion, and are supposed * “ The polypi are most intimately and inseparably connected with the axis by three parts of their body, and are only digestive sacs or mouths developed by the axis, as in all other zoophytes, for the nouiishment of the general mass. By the axis of a zoophyte, I understand every part of the body excepting the polypi, whether of a calcareous, horny, or fleshy nature. The exact matliematical ar- rangement and forms of the cells of Flustrae is incompatible with their existence as separate and independent beings, but is quite analogous to what we are ac- customed to observe in Cellari®, Sertulariae, Plumulariie, and many other well- known compound animals.”—Grant in Edin. New Plul. Journ. iii. 116. See also Blainville, Man. d’Actinologie, ]». 99. t The figure, for which 1 am indebted to my friend Mr William Scott, repre- sents the polypes of Flustra membranacea in a state of retraction. J For a history of this discovery, written with great learning and impartiality, Fig. 2.](https://iiif.wellcomecollection.org/image/b21910479_0054.jp2/full/800%2C/0/default.jpg)
