. The elasmobranch fishes . pharyngeal?) regions; (2) a long middle portion lined with entoderm which includes in the adult the segments from the oesophagus to the end of the rectum, and finally (3) a posterior proctodeimi or cloacal area also lined with ectoderm. From these simple beginnings the complex tract results. As growth proceeds a series of dilations and con- strictions divides the tract into parts characteristic of the adult. These we shall next consider in order. Buccal Cavity The mouth in Elasmobranchs is a large crescent which is usually ventral, although in certain types it is te
. The elasmobranch fishes . pharyngeal?) regions; (2) a long middle portion lined with entoderm which includes in the adult the segments from the oesophagus to the end of the rectum, and finally (3) a posterior proctodeimi or cloacal area also lined with ectoderm. From these simple beginnings the complex tract results. As growth proceeds a series of dilations and con- strictions divides the tract into parts characteristic of the adult. These we shall next consider in order. Buccal Cavity The mouth in Elasmobranchs is a large crescent which is usually ventral, although in certain types it is terminal, in position. It is bounded by membranous folds or lips and leads into a voluminous buccal cavity. The floor of the buccal cavity is raised up into a heavy fold, the "tongue," which in some forms (Lamna) is well developed; in others it is less pronounced. The buccal cav- ity is lined with a smooth or papillated mucous membrane (Mustelus, Scyllium, Chlamifdoselachus), the cells of which secrete mucin; but it is devoid of all glands which are characteristically present in higher forms. Perforating the lining of the cavity are two structures which, although differing in form, are essentially identical: the stomodeal denticles and the teeth. The former we have considered in Chapter II, page 38. The latter may be discussed here more in detail. TEETH The teeth^ characteristic of the Elasmobranchs are of two types: sharp or prehensile teeth and pavement or crushing teeth. Between these extremes multitudes of patterns, more or less complex, occur. In the early stages the two types are essentially alike, but as development proceeds each takes on its specific character. The general mode of devel- opment we may examine before considering the types further. A sagittal section made through the lower jaw of Spinax niger by Laaser (fig. 125) shows the ectoderm sinking in to form a dental ridge (). In this ridge several tooth germs are developing, and cells are collect- in
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