. Elements of Comparative Anatomy. . 360 COMPAEATIVE ANATOMY. to its function; this function is to seize and comminute tlie food of the animal. The apparatus, which forms the chief portion of this arrangement, is a cuticular membrane which rises up from the lower wall; on it there are small teeth or hooks, which are directed back- wards and arranged in transverse rows. The teeth vaiy very greatly in arrangement (Fig. 191, a h c d), form, and number; they are not only different in the larger di- visions, but also in the orders and families, and even in the species, so that the genetic affi- nit
. Elements of Comparative Anatomy. . 360 COMPAEATIVE ANATOMY. to its function; this function is to seize and comminute tlie food of the animal. The apparatus, which forms the chief portion of this arrangement, is a cuticular membrane which rises up from the lower wall; on it there are small teeth or hooks, which are directed back- wards and arranged in transverse rows. The teeth vaiy very greatly in arrangement (Fig. 191, a h c d), form, and number; they are not only different in the larger di- visions, but also in the orders and families, and even in the species, so that the genetic affi- nities of the groups is indicated by the form of these parts ; and for this reason they have been much used in classification. As Fig. 191. A row of denticles from ihe f ™}e they consist of a median radulaofLittorinalittorea. a Median. lougltudmal rOW (ft), with whlch h c d Lateral denticles (after Gray). symmetrically- arranged lateral denticles are connected. The organ formed by the whole of these booklets is known as the radula. In many (Turbo, Patella) it projects some way into the coelom, and is enclosed by the sac-like sheath formed by a diverticulum of the wall of the oesophagus; it may even be longer than the body itself. This organ may become very broad, and reach to the sides of the pharynx. In the Heteropoda it is so far more highly developed that the outermost uncini of the transverse rows may not only be very long, but also be articulated in such a manner as to be movable. When, therefore, the radula is protruded, these teeth are erected, and when it is drawn back they come together like pincers, and so act as seizing organs. There are special muscles for moving the radula which, with the cartilage on the wall of the pharynx (p. 361), hel^Dto increase the size of this organ (Fig. 200, B). The size of the radula is therefore closely correlated with that of the buccal mass. This organ is found in all divisions of the Mollusca excepting the Lamellibranchiata,although i
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