MUSCLES OF CERATODUS 355 



The Constrictor laryngis consists of short muscle-cells 

 encircling the larynx in a horizontal plane. In transverse 

 sections they are most obvious as such just in front and behind 

 the larynx. The Constrictor laryngis gradually increases in 

 size, and in the adult forms a muscle of many fasciculi which 

 are penetrated by the dilatator fibres of the Constrictor 

 oesophagi. 



Innervation . — In a 27 mm. embryo the N. intestinalis x. 

 passes back dorso-lateral to the oesophagus just within the 

 upper edge of the ventral constituent of the Sphincter oesophagi 

 et laryngis. At the anterior edge of the Constrictor oesophagi 

 it gives off a ventral branch which passes to the larynx, the 

 Constrictor oesophagi and Constrictor laryngis. 



The oesophageal and laryngeal muscles of Ceratodus thus 

 resemble those of Menopoma in that they are differentiated 

 from cells which are proliferated from the splanchnic layer 

 of the coelomic epithelium. 



Pinkus described in Protopterus a fine twig from the N. intes- 

 tinalis X. to the mucous membrane of the pharynx and larynx. 

 This is the homologue of the laryngeal branch of the N. intes- 

 tinalis x. in Ceratodus. 



The evidence hitherto available suggests the probability 

 that the Dilatator and Constrictor laryngis, and the Dilatator 

 laryngis (and also the Constrictor oesophagi if microscopical 

 examination shows it to be present) of Lepidosiren will be 

 found to be developed from the cells which were shown by Agar 

 to be proliferated from the pericardio-peritoneal ducts. 



But, however this may be, comparison of the adult anatomy 

 of Ceratodus, Protopterus, and Lepidosiren shows that the 

 laryngeal muscles of Ceratodus — consisting as they do of a 

 Constrictor laryngis and the (unseparated) fibres of the Con- 

 strictor oesophagi which act as a dilatator — are the most primi- 

 tive existing in vertebrates. 



I have the pleasure of thanking Dr. Bancroft for the 

 embryonic and adult stages of Ceratodus, and the Bristol 

 University Colston Society for defraying the expenses incurred 

 in the investigation. 



