The only other special organs of sensation possessed by the Clio are the tentacula ; but these will be best described in connexion with the head to which they are appended.
Head-cowls and Tentacula.— The structure of the head of Clio is very remarkable; and, in its general characters, cannot be more ap propriately described than in the words of Pallas.* " Caput contractum subgloboso didymum est, lobo vel utroque vel alterutro, imo quandoque neutro, antice papilla cornea (tentaculum) acuta, mucronata. Qui lobi sunt proprie prmputia duo (the head-cowls) crassa, carnea ; hemisphwrica, contractilia, basi coad unata, e quoruno interiore latere emergunt tentacula (head-cones) tria carnosa, conica, sequalia qum ori utrinque adstant et contracta prmputio tota delitescunt." The above description will, however, be better understood by a reference to the ac companying figures, in which the structures above mentioned are delineated on a large scale. In fig. 110. 11 the head is represented, seen from the ventral aspect with the head-cowls (a, b) dosed together, concealing all the other organs except the tentacula, one of which (k) is seen protruding through an opening in the left cowl, that of the opposite side being re tracted —while in fig. 110. 10 the head-cowls are shown partially folded back, so as to dis play the conical appendages (head-cones) which the cowls enclose and protect.
Each of the cowls (lobi, Pallas ; buccer, Fabricius) seems, when more closely ex amined, to be composed of two spherical parts intimately conjoined, of which the anterior (fig. 110. 11, a a) is the smaller, and the posterior (b b) the larger. The posterior spherical por tions are continuous with each other ; they enclose a large c,avity, which is, in its widest part, filled up by the penis ; but, in its nar rower and median part, contains the parts of the mouth — the cesophagus and the salivary apparatus. The smaller or anterior spheres, on the contrary, are only produced by the folding of the skin over the head-cones, and disappear when these organs are protruded. In the fore part of each of the anterior sphe rical portions of the cowl is a little flat sur face, in the middle of which may be observed either the tentacle ( fig. 110. 10 and 11, k), or the orifice (fig.110. 12, /), through which it is protruded : the two flat surfaces are separated from each other when the cowls are closed by a longitudinal fissure (p), the margins of svhich form two prominent lips (o o).
The lateral tentacles (k) are cylindrical, smooth, and terminated by rounded extremi ties. They. are hollow, and in their interior, three longitudinal bands of muscle and a nerve of considerable size are distinguishable, so that they can be retracted in the same manner as the horns of a snail, nothing re maining externally to indicate their position, except the hole through which they are pro truded. When thus inverted the tentacles are found lodged in the cavity of the head, with their apices directed inwards.
The two smaller spheres of the hood or cowl are separated from each other by the longitudinal fissure (fig. 110. 11), which Fabri cius, very inappropriately, called the mouth, although, at the same time, he was acquainted with the real mouth, and recognised it as such. This vertical fissure occupies the entire top of the head, and is continued for some distance both on its upper and under surface, or, more properly speaking, the real head is buried deeply in the interspace be tween the two cowls, and when these are sepa rated from each other, the following parts are seen situated between them : in the centre of the floor of the fissure is the vertical opening of the mouth (fig. 110. 13, u), between which and the borders of the hood (g), are the cres centic spaces (r), in which are situated the conical appendages to the head already men tioned, and which are represented protruding from between the margins of the hood in fig. 110. 10 (s).
Conical Appendages to the Head. — The co nical appenda‘res to the head (Kopfkegel, Esehrieht), when fully expanded, form a kind of star round the mouth (fig. 108. 3, s), and were erroneously styled by Fabricius " soft teeth" (" suntque dentes hi 'wiles subcrus tacei"). It is to Eschricht we are indebted for a knowledge of the real nature of these wonderful organs, the structure of which is unparalleled in the animal creation. It has been already noticed that these conical bodies are of a red colour in the recent animal, and, when they are protruded, it is easily discover able with a lens that this colour depends on the presence of numerous separate coloured points distributed over their surface. When still further maanified, these points show themselves as crosely aggregated spots, ar ranged with great regularity upon the exterior of the cone. Upon a rough calculation there may be about three thousand of these spots upon each conical appendage, each of which, when closely examined, under favourable cir cumstances, assumes very much the appear ance of the polype-cell of one of the Sertularian polypes, and exhibits a structure which is truly admirable. Each little spot consists, in fact, of a transparent sheath, enclosing a cen tral body, composed of a stem terminated by a kind of tuft, which last can be protruded at times beyond the margin of the sheath. When viewed laterally (fig.111.14) it is appa rent that this central body consists of several filaments or tubes, every one of which ex pands at its extremity into a dilated portion, terminated by a little disc (fig. 111.15), and about twenty of these are enclosed in each sheath. The conical appendages to the head of a single Clio are, therefore, furnished with (20 x 3000 x 6) about three hundred and sixty thousand of the stem-supported discs in question.