210 AN INTRODUCTION TO ZOOLOGY 



by the pectoral fins, and the dorsal, ventral and pelvic fins act as 

 steadying or equilibrating organs. 



The head is somewhat flattened, and of a triangular shape, 

 ending in a bluntly rounded snout. On the sides of the head are 

 situated the eyes. They are provided with upper and lower lids, 

 the latter are able to be pulled right up over the eye. The external 

 nares are two approximately circular openings on the ventral 

 surface of the head, a short distance from the front of the snout. 

 Each leads into a hollow cavity, the nasal sac or olfactory organ, 

 which has no internal narial passage such as we find in the frog. 

 On the other hand, the nasal sac communicates with the mouth by 

 means of a deep groove, the naso-buccal or oro-nasal groove, which 

 is partly covered by a flap-like extension of the skin termed the 

 fronto-nasal process. This is a very primitive condition, and one 

 that is met with in the embryos of the higher Craniates. It is a 

 failure in the complete transformation of this into an internal nasal 

 passage in man that leads to the condition known as cleft-palate 

 or hare-lip. Just behind the olfactory organs is the large mouth of a 

 crescentic shape, and provided with a formidable armament of many 

 rows of strong pointed teeth. A short distance behind the eye is a 

 small circular orifice, the Spiracle, which leads into the pharynx. 

 In spite of their anterior position and appearance these perforations 

 are homologous with true gill clefts, and their development shows 

 that they are in reality the foremost of the series. Indication of the 

 fact that they were at one time similar to the other clefts is to be 

 found in the presence of a tuft of rudimentary gill filaments, the 

 pseudobranch, situated on their anterior wall. They appear to be 

 mainly concerned with the intake of water for respiratory purposes. 

 It is the spiracular cleft, lying as it does in close proximity to the 

 auditory capsule, that takes part in the formation of the ear passages 

 of higher vertebrates. 



A short distance further back on each side are a series of five 

 vertical slits decreasing in size from the front, these are the external 

 gill clefts. When they are opened it will be seen that they lead into 

 a corresponding series of branchial pouches. The anterior and 

 posterior wall of each pouch, save the hinder wall of the last, is 

 thrown into a series of radiating folds, the branchial filaments or 

 gills, which are delicate and very vascular, so that they appear 

 bright red in life or in a freshly killed fish. 



There remain for mention two apertures on the dorsal surface of 

 the head, so small as to be practically invisible, although their 

 position may be demonstrated in a fresh specimen by pressing the 

 back of the head which causes drops of a milky fluid to be exuded. 

 They lie in the mid-dorsal line just about on a level with the front of 



