HYOBRANCHIAL APPARATUS OF SPELERPES 547 



cytes, and new fibers of the abdominohyoideus. By stage IV 

 the entire abdominohyoideus and sternohyoideus muscle are 

 quite definitely formed and await only final consumption by the 

 phagocytes of the few remaining degenerate fibers and the filling 

 out to their adult capacity of the new ones already formed. 



While the transformation of the thoracicohyoideus into the 

 abdominohyoideus and sternohyoideus involves the two prin- 

 ciples — development of new fibers and the degeneration of old — 

 the other muscles exhibit the one principle or the other, and not 

 both. Thus the first alone is found in the geniohyoideus laterahs, 

 which is also present in stage I as a proliferation of cells from the 

 dorsolateral surface of the anterior end of the larval genio- 

 hyoideus, or geniohyoideus medialis as it must now be called. 

 In this stage the new fibers are very short and are inserted into, 

 the mucous membrane of the pharynx (fig. 29). They now 

 grow rapidly in a posterolateral direction, so that by stage II 

 they approach the ceratohyal; by stage III are inserted on it 

 for a considerable distance, and by stage IV have very nearly 

 their adult insertion. The second principle is found in the 

 adductor, constrictors, first three levators, and the third de- 

 pressor, which gradually degenerate, as the epibranchials which 

 they regulated break down. The fourth depressor, however, 

 and the fourth and fifth levators, which are hyobranchial muscles 

 only in their comparative morphology, but are physiologically 

 pharyngeal muscles used in respiration, change merely by an 

 increase in size and loss of the identity of their component parts. 

 This last fact brings out quite forcibly the difference between 

 the raphe separating the fourth levator and depressor which has 

 been proved to be a vestigial organ (Goppert, '91, H. H. Wilder, 

 '96), and the raphe separating the moieties of the intermandibu- 

 lares, which developed secondarily at metamorphosis, evidently 

 in response to an important physiological need, and which is 

 therefore not vestigial, but progressive. 



Perhaps the most noticeable and quickly carried out of the 

 changes in the musculature at metamorphosis is furnished by 

 the ceratohyoideus externus and internus. The former, which 

 , was by far the largest of the intrinsic larval muscles, but was in 



