128 



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 

 (d.r.). In this ridge several tooth germs are developing, and cells are collect- 

 ing at the papilla (p.) to form still another tooth. In the tooth germs which are 

 more mature than those just mentioned the papillae have gro\\ai outward 

 carrying caps of epidermis, the enamel organ (e.o.), over them. In their for- 

 mation the teeth are much like the saw tooth already studied (p. 35) . 



Teeth thus formed in other Elasmobranchs may be long and fang-like, 

 1 Tor bibliography on the teeth, see Chap. II. 



B 



Fig. 126. Tooth pat- 

 terns. A. ChJamy- 

 doseJachus. (From 

 Bos©.) B. Mylio- 

 batis. (From Gar- 

 man.) 



