610 



CILIA. 



more correctly described the course of the cur- 

 rents : the foregoing description is in a great 

 measure taken from his memoir. Since then 

 several others* have made similar observations, 

 among whom we may mention Raspail as 

 more particularly deserving of notice, though 



figure), and they are clearly to be seen on 

 the whole internal surface of the rectum (6). 



I have nowhere more clearly seen the above- 

 mentioned phenomena than in a zoophyte, 

 whose polype, though differing somewhat from 

 the first form, may yet be referred to it. This 



B 



Polype of a Flustra 

 in its cell. 



he here, as in other cases, denies the existence zoophyte (fig. 293, A, B) has a creeping stem 

 of cilia. 



b. Marine Polypi. The polypi of marine Fig. 293. 



Zoophytes, on which observations relating to 

 the present subject have been made, may for 

 our purpose be conveniently arranged under 

 three principal forms. 



The first form of polype (fig. 292) is found 

 in Flustrae and cellular 

 polypi generally ; it ex- 

 ists also in some spe- 

 cies which have been 

 classed among the Ser- 

 tularias, and probably 

 prevails very extensively 

 in different tribes of 

 Zoophytes. The body 

 (a, b, c), which is gene- 

 rally contained in a cell, 

 is bent on itself, some- 

 what like the letter Y or 

 V; the one branch (a) 

 being the mouth and 

 throat, the other (b) the 

 rectum opening by an 

 anus, and the middle 

 part (c),which is of a dark 



and often of a brown co- 

 lour, being the stomach probably with some 

 accessory organ. The mouth is surrounded 

 with a variable number of long straight ten- 



tacula or arms, fringed on both of their lateral 



margins with cilia. When the arms are ex- 

 panded, the cilia are thrown into rapid motion, 



which has the appearance of undulations pro- 

 ceeding along the fringes, upwards on one side 



of the arm or from its root to the point, and 



downwards on the other. While the cilia are 



thus moved, they produce currents in the water, 



as described in the Fresh-water Polype, and 



here also the currents in all probability serve 



for respiration and the prehension of food. 



Besides these motions in the water in the 



neighbourhood of the tentacula, a revolving 



motion of particles is observed within the 



body : small particles of extraneous matter 



which enter the throat are moved round 



within it ; and the contents of the stomach 



and rectum undergo a very singular revolving 



motion round the axis of the cavity. These 



internal motions, Dr. Grant conjectured, might 



be owing to internal cilia; and I have been 



able to satisfy myself of the actual existence 



of such internal cilia, by means of a Wollas- 



ton's doublet of one-thirtyfifth of an inch focus ; 



they are very evident in the throat; in the 



stomach they are most distinct in the part 



adjoining the rectum (indicated by d in the 



* Vaucher, Bull, de la Soc. Philom. An xii. ; 



Raspail, Mem. de la Soc. d'Hist. Nat. de Paris, 



for 1827; Mcyen liber Polypen, Isis 1828, p. 

 1225. 



(a, a), which adheres to shells, or twines round 

 the stems and branches of other zoophytes, 

 (as b in the figure) ; the polypes are supported 

 on soft pliable fleshy stalks (c), which the crea- 

 ture moves from time to time ; their body (d, 

 and B more magnified) is bell-shaped and 

 consists of a transparent brownish skin or 

 envelope containing the mouth and throat (e), 

 the stomach (g), and rectum (A). The mouth, 

 or expanded aperture of the animal, is sur- 

 rounded by a prominent lip or border (i, f), 

 to which the arms are attached. Cilia are 

 distinctly visible on the arms, and within the 

 mouth and stomach ; they are moved very 

 briskly, and small extraneous particles indi- 

 cating currents in the water are hurried onwards 

 towards the arms, as pointed out by the arrows 

 at k, k ; many of these particles descend along 

 the inner side of the arms to their base, as 

 shown by the dotted arrows o, o, o, and thence 

 into the cavity of the mouth, from which, after 

 being moved about for some time, the greater 

 number are thrown out. It would seem that 

 the particles of food or other solid matter, 

 after being conveyed to the inside of the arms, 

 take then a different course from the stream 

 of water. The latter passes inwards between 

 the arms, and issues from the middle of the 

 irregular circle which they form (as at m, ni), 

 carrying with it such solid matters as are not 

 arrested on the arms; but the bodies which 

 enter the mouth are slowly carried along the 

 inside of the arms (as at o, o), and in close 

 contact with them till they reach their base. 

 The motions of the contents of the stomach 

 and its cilia appeared as in the Flustrae. 

 I could perceive none in the rectum. Mr. 

 Lister has described the same phenomena in a 

 zoophyte closely resembling this one in the 

 structure of the polypi, but differing in the 

 character of the stem.* 



* Phil. Trans, for 1834, p. 385. 



