HYOBRANCHIAL APPARATUS OF SPELERPES 541 



-edge of the basibranchial, for a short distance, but most of them 

 bend around ventrolaterally and partially enwrap the second 

 ceratobranchial from the ventral side. The muscle then con- 

 verges again, dorsal to the first ceratobranchial, and, considerably 

 diminished in size, continues forward in the dorsolateral groove 

 of the basibranchial, within the tongue stalk, nearly to the 

 anterior end of the cartilage. Finally it converges dorsolaterally 

 within the tongue substance and inserts through a tendon into 

 the 'little horn.' 



Near this lingual insertion of the abdominohyoideus is a 

 muscle, not present in the larva and not very fully developed 

 even in the adult. This is the hyoglossus, which consists merely 

 of delicate fibers which arise from the lingual cartilage and 

 anterior tip of the basibranchial and radiate out through the' 

 tongue substance, some to be inserted on the kittle horns' and 

 some in the mucous membrane of the disk. 



The ceratohyoideus externus has disappeared entirely, but the 

 simple small ceratohyoideus internus of the larva has become, in 

 the adult, by far the most unique and highly specialized of the 

 entire group. It now arises from the ventral surface of the 

 ceratohyal, and for a short distance its fibers run posteriorly in 

 a straight longitudinal direction. Soon, however, they begin 

 to curve around from the medial edge of the ceratohyal to a 

 position lateral to its outer edge, and back again to a medio- 

 posterio direction. At this point the muscle forms a kind of 

 pocket which opens medially and in which lie the distal ends of 

 the first and second ceratobranchials. Posteriorly, the edges of 

 the pocket converge, and the muscle, added to by new fibers 

 which develop at metamorphosis, becomes wound around the 

 epibranchial in a complicated spiral, the more exact structure 

 of which I have not worked out. The muscle continues a little 

 posterior to the end of the epibranchial, forming a kind of closed 

 sac which, on shortening of the spiral muscle, evidently acts as 

 a bulb and squeezes the epibranchial almost entirely out of the 

 pocket as a means of pushing out the tongue. 



Conspicuous in a dorsal view^ of the floor of the mouth, when 

 the posterior edge of the tongue is lifted forward, even before 



