The Mechanism of Pulmonary Eespiration in Ainphibians with Gill Clefts. 71 



but the adjoining margins of the second and third clefts are furni- 

 shed with separate folds. 



In the larval Salamanders accessory structures for closing and 

 protecting the gill clefts are strongly developed (Figs. 1 and 2). In 

 all of the forms examined, including Triton alpestris, Salamandra 

 maculosa, Salamandrina perspicülata and Amblystoma tigrinum, I 

 find essentially the same conditions. All of the clefts, excepting the 

 first, or hyobranchial, are protected internally by conspicuous gill 

 rakers, similar in form and structure to those of Siren. The dorsal 

 half of the first cleft is protected only by the operculum ; the ven- 

 tral half is guarded, in addition, by a membraneous fold which is 

 attached to the inner surface of the hyoid arch. Externally all gill 

 clefts open into an opercular Chamber and are protected by an 

 opercular membrane (gular fold), which arises in front of the first 

 cleft. The muscle of the operculum acts as a constrictor of the 

 pharynx and assists in closing the clefts. The operculum is followed 

 by a series of smaller membranes which are attached, one to each 

 of the branchial arches. By the union of adjacent membranes below 

 the clefts, the latter are completely surrounded except at their dor- 

 sal angles, where the membranes terminate at the base of the gills. 

 Along the liue of attachment to the branchial arches the membranes 

 have a considerable thickness which may equal that of the arches 

 themselves, but they become thin toward their free edges which lie 

 near the posterior margin of the operculum. These membranes are 

 somewhat elastic structures which follow the movements of the gill 

 arches, diverging from each other when the clefts are open and 

 overlapping more or less closely when the clefts are closed. In the 

 latter position the contact of their thickened bases makes the closing 

 of the clefts secure. That the closing of the clefts is effective during 

 the act of pulmonary respiration has been repeatedly demonstrated 

 by observations on the large larva of Amblijstoma tigrinum. 



In the frog larva the gill clefts can not be wholly closed but 

 the escape of respiratory air can be prevented by other means. 

 According to Schulze (1892) the posterior part of the peribranchial 

 Chamber can be closed by two muscles, the subbranchialis and dia- 

 phragmatico-praecordialis. 



The Closing of the Nasal Passages. 

 In Ampkiuma and Cryptobranchus allegheniensis the external 

 naris leads into a short vestibule whose inner portion can be closed 

 and opened by a smooth muscle mechanism (Mrs. Wilder, • 1909). 



