270 CONTRIBUTIONS TO THE NATURAL HISTORY 



nervous mass at the base of the eyes present a structure not very different from that of the 

 radiating cones in the bulb of Hippocrene ; and I cannot help finding in this analogy an 

 additional argument in favor of the opinion which considers the marginal specks of Me- 

 dusae as a visual apparatus. The supposition that the eye-specks of Radiata may be or- 

 gans of hearing seems to me, indeed, entirely set aside by the peculiar arrangement of 

 the sensitive bulb in this genus. 



The tentacles themselves are transparent threads, capable of extraordinary dilatation 

 and contraction, although they consist exclusively of cells. When fully elongated (Plate 

 I. Fig. 2), they appear like very thin undulating threads stretched out in a radiating 

 manner ; the middle and longer ones, however, always rise above the lateral ones, which 

 are shorter. These bunches of tentacles seen from above or below appear like a brush 

 of threads ; but when seen in profile, it is plain that the middle ones are arched over the 

 lateral ones, and hence the very different appearances which the animal with extended 

 tentacles may present in different positions, when the tentacles are more or less raised, as 

 may be judged from the appearance of Fig. 1, 2, 5, and 6, Plate I. The more the tentacles 

 contract, the thicker they appear, and in their contractions they assume also a more un- 

 dulating or curved appearance, as may be seen in Fig. 3 and 4, in the latter of which 

 they are so much contracted as to form a short bunch of thick threads round the eye- 

 specks ; and in this contracted state the tentacles are again straighter than in a half-ex- 

 panded position, as may be noticed in Plate I. Fig. 4. The dilatation or contraction of 

 the tentacles does not take place through their whole length simultaneously, for the base 

 may be elongated and the upper portion still contracted, or the extremity may be elongat- 

 ed and the base in a contracted state ; or the middle portion alone may be swollen, and 

 the base and tip elongated. And these contractions and dilatations take place in rapid 

 succession, the tentacles being rather active ; but though displaying such an activity, and 

 such a power of dilatation and contraction, — evidently voluntary movements, — they con- 

 sist wholly of cells of two different kinds ; one row of large cells (Plate II. Fig. 2, 3, 4, a) 

 forming the main trunk of the tentacle, the cells being either arranged in single rows, or en- 

 croaching upon each other irregularly ; while much more numerous smaller cells (6, b) form 

 a uniform coating over the whole tentacle. The contractile organ of the tentacle doubt- 

 less consists of the inner row of cells ; for these change their shape considerably in the 

 different forms of the tentacles. When fully contracted, as in Plate II. Fig. 13, they are 

 narrowed into transverse fibres, and seem to be transverse muscles. But when elongated 

 they are like square masses, the longitudinal diameter of which is scarcely half their 

 transverse diameter, as in Fig. 3, a. Both forms, though less contracted, may be ob- 

 served in Fig. 14, where the cells are broader in the elongated part, c, c, of the ten- 

 tacle, and narrower in its contracted portion, 6, b. 



