GILLS. 139 



Lepidosiren and Protopterus a part of the branchial arches is 

 gill-less. In Polyodon, Ceratodus, and Polypterus, an opercular 

 gill is absent, the two former having four complete gills, the 

 latter three and a haK only. Spiracles are still in some 

 Ganoids present, viz. in the Sturgeons and Polypterus. In aU 

 the Ganoids an osseous giU-cover is now developed. 



In the Teleostei the giUs with their supporting branchial 

 arches lie in one undivided cavity ; more or less wide clefts 

 between the arches lead from the pharynx to the gills, and a 

 more or less wide opening gives exit to the water after it has 

 washed the gills. The interbranchial clefts have sometimes 

 nearly the same extent as the branchial arches ; sometimes 

 they are reduced to small openings, the integuments stretch- 

 ing from one arch to the other. Sometimes there is no cleft 

 behind the fourth arch, in which case this arch has only an 

 uniserial gill developed. The gill-opening likewise varies 

 much in its extent, and when reduced to a foramen may be 

 situated at any place of the posterior boundary of the head. 

 In the Synibranchidm the gill-openings coalesce into a single 

 narrow slit ia the median line of the isthmus. In the 

 majority of Teleosteans the integument of the concave side of 

 the branchial arches develops a series of horny protuberances 

 of various form, the so-called gill-rakers. They are destined 

 to catch any solid corpuscles or substances which would be 

 carried into the gill-cavity with the water. In some fishes 

 they are setiform, and form a complete sieve, whilst in others 

 they are merely rough tubercles, the action of which must be 

 very iucomplete if they have any function at all. 



Most Teleosteans possess four complete giUs, but fre- 

 quently the fourth arch is provided with an uniserial gQl 

 only, as mentioned above, or even entirely gill-less. The 

 most imperfect giUs are found in Malthe, which has two and 

 a half giUs only, and in AmpMpnous michia, in which one 

 small gni is fixed to the second arch. 



