536 LOUISE SMITH 



ventral half of digastricus pharyngeus or pharyiigobranchialis 4) 

 arises from the rudimentary fourth epibranchial and is inserted 

 like the third, in the midline. The fifth, like other parts associ- 

 ated with the larynx, is lacking altogether. 



ADULT CONDITION 



1 . Hyohranchial skeleton 



In response to its changed functions, the hyohranchial skeleton 

 of the adult has lost almost all resemblance to that of the larva 

 (figs. 17 and 18). From a rather heavy structure with all its 

 parts firmly bound together, it has become a very delicate one 

 with a lack of firm articulations, and thus well adapted to the 

 complexity of motion characteristic of it at this time. This 

 great range of motion makes description rather difficult for the 

 relationships when the tongue is withdrawn are much changed 

 when it is extruded. In the following discussion, I describe the 

 skeleton in the normal resting position with the tongue well 

 withdrawn, unless the contrary is definitely stated. 



Though the first basibranchial is still of great importance as 

 supporting the tongue and the cartilages which govern its motion, 

 it is no longer the pivot which binds together all the other parts 

 of the visceral skeleton. Through the loss of the articulation 

 betw^een the ceratohyals and the basal piece and the breaking 

 down of the branchial plate into the first ceratobranchials and the 

 OS thyreoideum, the hyohranchial skeleton may be arbitrarily 

 divided into two portions, a central one associated with the first 

 basibranchial and an outlying one, consisting of separate pieces. 



Of the latter, the os thyreoideum is the most superficial (fig. 

 17). This is a smaU, median, crescentic bone, just beneath the 

 ventral integument, and, despite its minuteness, it is especially 

 significant as the basis of attachment of three important muscles. 

 It is formed by the lateral growth and ossification of the free end 

 of the second basibranchial and is unique as being the only ossified 

 part of the entire hyohranchial apparatus. 



The ceratohyal shows perhaps more change from its larval 

 appearance than any other single cartilage. It now lies with its 



