STRUCTURES OF TELE OS T 



143 



laginoLis brain case is replaced by many definite osseous 

 elements. The floor and roof of the skull, the face region, 

 jaws, gill arches, and their protecting parts, are all encased 

 by an elaborate series of membrane bones ; these, however, 

 must be noted as deeply embedded in the body tissue, 

 DCR, DN, A, Q, FT, SM, BR, O. The membrane bones 

 of the jaw rim — -maxillary, premaxillary, and dentary, MX, 

 PjWX, DN — bear teeth, and are especially characteristic 

 of the Teleostpmes ; those overlapping and protecting the 

 gill arches ifiA), O, /O, PO, SO, usually four in number, 

 are also characteristic of the group. The skull is hyostylic. 

 As to the visceral parts. The gill arches, GA, are 

 reduced in number, usually widely bent backward, and 

 closely crowded together ; their gill filaments are enlarged 

 and specialized. The heart lacks the arterial cone with its 

 transverse series of valves ; in its place a stout bulbus, B, 

 forms the base of the aorta. The digestive tract is tubular, 

 long, and coiled ; its intestine, G, lacks a spiral valve, and 

 terminates at the body surface, AA^, not in a cloaca ; its 

 glands include a series, often great in number, of pyloric 

 caeca (pancreas). An air-bladder, AB, is present, which 

 may, or may not, retain its communication with the gullet. 

 The ovary, with its many small eggs, and the kidney, dorsal 

 to it, have often a common external opening in a urino- 

 genital papilla, [/G, in either side of which abdominal pores 

 may occur. The nervous system and sense organs (pp. 275, 

 2yy) have many peculiarities : the roof of the fore brain is 

 non-nervous ; the nasal openings appear in the dorsal side 

 of the head, A/^O, and are separate; the eye has specialized a 

 vascular, nutritive structure, \\\.q processus falciformis, pro- 

 jecting from the region of the entrance of the optic nerve 

 into the vitreous cavity of the capsule ; the optic nerves 

 cross in passing to the eyes, but their fibres do not fuse. 



