MORFHOJ-OflY OF CJJAXIAL MUSCLES IX SO.AIK VKKTEBKATES. 259 



The sim])le.st and probably primary condition of the 

 muscles developed Ijetvveen tlie levatores above and. the 

 hypobranchial cranial muscles below is a series of Mm. 

 mai'ginales, as found in Anuran larvae and Ceratodns. In 

 Teleostomi these may unite with the (lateral halves of the) 

 transversi ventrales. In Scylliuni they undergo a quite 

 special development, which is not found iu any other group. 



The above comparisons suggest that the probable primitive 

 condition of each of the branchial myotomes was, from 

 above downwards, a levatoi-, a M. marginales, an inter- 

 arciialis ventrales, aud (the lateral half of a) transversus 

 ventralis. 



(Esophageal, Piiaryngeal, and Laryngeal Muscles. 

 — The term "phai-ynx" is employed by writers in two senses, 

 either restricted in meaning to the lu-anchial region of the 

 alimentary canal, or including this and the next succeeding 

 portion. Iu this paper it is used in the first sense. 



The oesophagus is surrounded by a circular muscle, the 

 constrictor, which is derived from cells given off from the 

 splanchnic layer of the calomic epithelium. 



No circular muscles are present in the branchial region of 

 Selachii, Acipenser, and. Ceratodus, but are present, in the 

 form of transversi dorsales, in Teleostomi (Vetter), Amia 

 (Allis), aud Lepidosteus (Wiedersheim), and are formed by 

 the constrictor of the oesophagus extending forwards, 

 dorsally, into the branchial i^egion. In Polypterus 

 senegalus the fore part of the oesophageal constrictor 

 slightly overlaps the branchial region dorsally, but the 

 transversely directed fibres are not attached to any branchial 

 bar. In Lepidosteus and Amia^ the conditions are com- 

 plicated by the presence of a dorsal larynx. The dorsal 

 larynx of Lepidosteus is formed, in 8 mm. embrj'os, as a 

 solid median upgrowth from the then solid oesophagus just 

 behind the branchial region. The oesophageal constrictor 

 (constrictor pharyngis of AViedersheitn) is formed from cells 



' The adult condition of the dorsal larynx and its muscnlatnre lias 

 been fully descril.ied by Wiederslieini. 



