124 THE STORY OF FISH LIFE. 



great importance, and occurs in a considerable 

 number of the larvse or embryos of the lower 

 vertebrates. When the mouth itself is not 

 directly suctorial, as in the lamprey which we 

 have just discussed, it is associated with a more 

 or less well-developed suctorial disc. For in- 

 stance, in tlie tadpoles of the frog, there is a disc 

 of this nature situated behind — tailwards — of 

 the mouth ; in the larval lepidosteous, or bony 

 pike of America, there is a similar disc in front 

 of the mouth. The larval ascidian, or sea-squirt, 

 has an adhesive disc, also situated in front of the 

 mouth. Traces of this disc appear in the young 

 sturgeon. It is believed, from these facts, that 

 the ancestral vertebrates probably all had the 

 mouth bounded by a suctorial disc, w^bich is there- 

 fore a primitive organ of some importance. Of 

 this disc a part only is developed in modern 

 larvae or embryos, which may be either that part 

 bounding the front of the mouth, or that behind 

 — tailwards — of it. Furthermore, the mouth 

 itself was also probably suctorial in character, 

 like that of the young lamprey ; later, it became 

 further modified for biting purposes and de- 

 veloped jaws. The function of the disc is for 

 the purposes of attachment to weeds or other 

 objects, while the larva is at rest. The action of 

 these discs can be readily studied by anyone who 

 will take the trouble to collect a few young tad- 

 poles from the nearest pond during the spring 

 months. Artificially hatched, "bony-pike" all 

 attached themselves to the sides of the glass 

 vessel in which they were placed, by their discs. 

 The young sea-squirt soon after hatching attaches 



