FISHERV BULLETIN: VOL. 69, NO. 4 



completely separating the mouth from the phar- 

 ynx. Two ridges in the roof of the mouth cor- 

 responded to two grooves in the posterior margin 

 of the tongue. This structural correspondence 

 suggested that vertical positioning of the tongue 

 was a normal occurrence. The movability of the 

 tongue, as well as several other attributes de- 

 scribed below, can only be demonstrated with 

 specimens of Isistius that have not been fixed. 

 Observations of these anatomical characters 

 led to the hypothesis that Isistius is capable of 

 achieving a vacuum with its mouth on a smooth 

 surface. Concurrently with the retracted, verti- 

 cal positioning of the tongue, the lips protruded 

 completely around the mouth. The fleshy lateral 

 lips contained well-developed labial cartilages 

 that caused them to be semirigid and, when pro- 

 truded, to complete an ovoid of labial margins 

 in a single plane (Figure 6). Such a structure 

 in contact with a smooth surface enables the 



Figure 6. — A demonstration of the coraoohyoideus mus- 

 cles of Isistitis. Note also the protruded lips, the in- 

 ternal openings of the spiracles, and the mandibular 

 teeth. 



shark to form a vacuum when the spiracles are 

 closed and the tongue then retracted. 



In order to further determine if behavioral 

 retraction of the tongue was probable, dissec- 

 tions were made of the ventral surface of the 

 .shark ju.st posterior to the mandible. The 

 paired coracohyoideus muscles that insert on the 

 tongue were unusually large in Isistius (Figure 



Fir.URE 7. — Exposed coracohyoideus muscles of a large 

 whitetip shark, central Pacific. 



6). A comparative dissection of a large, fresh, 

 whitetip shark, Carcharhinus longimanus 

 (Poey) was made (Figure?); the cross section- 

 al areas in Isistius were estimated to be four 

 times those of the same muscles in the whitetip 

 shark, both relative to the total lengths of the 

 sharks. Pulling caudad on the exposed coraco- 

 hyoideus muscles of Isistius caused the tongue 

 to retract to the nearly vertical ])Osition noted 

 before; concurrently, the mouth gaped and the 

 lilJs protruded. The tongue of the whitetip 

 shark was not movable and pulling on the cor- 

 acohyoideus muscles did not retract it. 



Later I attemtited to repeat Fowler's experi- 

 ment by holding the mouth of a living Isistius 

 against the side of a gempylid fish. In this case, 

 the shark did not make a biting motion but. in- 

 stead, the spiracles closed, the head flattened 

 slightly, and an oral vacuum was formed by 

 means of which I was able to lift the gempylid 

 from the table with no other support. 



It seemed that the remaining evidence needed 

 to indict Isistius would be the presence of hemi- 

 spheroidal i^lugs of fish flesh in their stomach 

 contents. This evidence was found when two 

 IsistiN.<; caught subsequently on the same cruise 

 were found to contain single plugs of flesh of 

 aiipropriate size and shai)e. One of these jilugs 

 was from a relatively large fish, judging from 

 the thickness of the myomeres; the other was 

 from a squid. During a later cruise in the same 

 area, Reginald M. Gooding, fishery biologist, 



794 



