Gill slits are gills with individual openings rather than an outer cover. A gill is an anatomical structure found in many aquatic organisms Cartilaginous fish such as sharks, rays, sawfish, and guitarfish all have gill slits. Cartilage is a type of dense Connective tissue. It is composed of specialized cells called chondrocytes that produce a large amount of extracellular matrix Sharks ( Superorder Selachimorpha) are a type of Fish with a full cartilaginous Skeleton and a highly streamlined body Batoidea is a Superorder of cartilaginous fish containing more than 500 described species in thirteen families Sawfishes are a family of marine animals related to Sharks and rays. The guitarfish are a family Rhinobatidae, of rays. The guitarfish are known for an elongated body with a flattened head and trunk and small ray like wings Most have five pairs, but a few species have 6 or 7 pairs. Bony fish have an outer bony gill covering called an operculum. The operculum of a bony fish is the hard bony flap covering and protecting the Gills In most fish the rear edge of the operculum roughly marks the division between
The term "gill slits" also refers to the folds of skin in all vertebrate embryos. The skin folds in mammals, birds, and reptiles are gill slits, but the gill slits in embryonic fish develop into gills, while the gill slits in other vertebrates develop into the throat area and the bones in the ear. Skin folds are areas of Skin where it folds Many skin folds are distinct heritable anatomical features and may be used for identification of animal Species, while Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. Reptiles, or members of the class Reptilia are air-breathing Cold-blooded Vertebrates that have skin covered in scales as opposed to hair or feathers Gill slits exist in all vertebrates at some time in their embryo stage. When the vertebrate reaches a certain point in its life the skin folds fuse together and the bones form the trachea and ear bones. Except in fish where they form the gills themselves.
In the 19th century, gill slits of vertebrate embryos were erroneously thought to be actual gills, and thus evidence for the recapitulation theory. The theory of recapitulation, also called the biogenetic law or embryological parallelism, and often expressed as Ontogeny recapitulates Phylogeny
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