134 



THE ELASMOBRANCH FISHES 



possess groups of cuneiform secreting cells. By the beating of the cilia, food 

 caught in the mucous secretion is directed into the intestine. 



Marine (1913) has shown for Ammocoetes of the brook lamprey that the 

 groups of cuneiform secreting cells degenerate at the time of metamorphosis, 

 and that the follicles of the thyroid gland arise from certain areas in the walls 

 surrounding these columns. 



In the embryo of the Elasmobranchs the thyroid arises similarly as an 

 evagination of the pharyngeal floor, or as a solid block of cells in which a lumen 

 may form (Acanthias) . Either way, in development, it sinks deeper. While it 

 may retain its connection with the pharynx practically until the period of 

 birth, later than this all relation with the pharynx is usually lost and the 

 thyroid becomes a "ductless gland." 



Especially significant in this regard is the discover}' by Goodey (1910) that 

 in Chlaniydoselachus the duct (t.d., fig. 132) retains its connection with the 

 pharynx. In this form the aperture of the duct enters the pharynx, through 

 a perforation in the basihyal cartilage, and is lined with the pharyngeal epi- 

 thelium as is the endostyle of Amphioxus. Within this invaginated lining of 

 Chlaniydoselachus numerous scale-like structures are present, the remnants 

 of stomodeal denticles (s.d.). 



Oesophagus 



In most of the Elasmobranchs the segment of the digestive tract known as the 

 oesophagus {oe., figs. 173 and 175) is short. In some forms it is easily distin- 

 guished from the stomach liy its smaller 

 size, but in others it is wide and ]>asses al- 

 most imperceptibly over into the stomach. 

 Denticles which may be present on the 

 ])haryngeal lining cease more or less ab- 

 ruptly at the beginning of the oesophagus. 

 The mucous membrane lining the oesoph- 

 agus may be covered with long, finger- 

 like papillae as in Acanthias, but usually 

 it is thrown into numerous folds. Ante- 

 riorly these folds are low and regular; 

 posteriorly they may run transversely 

 marking a boundary between the oesopha- 

 gus and the stomach. Anteriorly, too, the 

 mucous membrane is similar to that of the pharynx, but posteriorly it consists 

 mainly of ciliated and goblet cells. 



A section of the membrane lining the oesoi^hagus of Squatina, by Petersen 

 (1908), shows the two main types of cells (fig. 133). One of these is of long 

 ciliated cells (cil.), among which are scattered the mucous cells (mc). The 

 mucous cells are extremely large and vacuolated, and each has its nucleus w^ell 

 toward the basal end of the cell. 



rig. 133. A section through the lin- 

 ing of the oesophagus of Squatina 

 (From Petersen.) 



cil., long ciliated cell; m.c, mu- 

 cous cell. 



