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Ileum, caecum and colon of rabbit, showing Appendix vermiformis on fully functional caecum The human vermiform appendix on the vestigial caecum. The appendix was once believed to be a vestige of a redundant organ that in ancestral species had digestive functions, much as it still does in extant species in which intestinal flora hydrolyze cellulose and similar indigestible plant materials. [10]
In humans, the vermiform appendix is sometimes called a vestigial structure as it has lost much of its ancestral digestive function.. Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. [1]
In fish, the arches support the gills and are known as the branchial arches, or gill arches. In the human embryo , the arches are first seen during the fourth week of development . They appear as a series of outpouchings of mesoderm on both sides of the developing pharynx .
Pharyngeal clefts resembling gill slits are transiently present during the embryonic stages of tetrapod development. The presence of pharyngeal arches and clefts in the neck of the developing human embryo famously led Ernst Haeckel to postulate that " ontogeny recapitulates phylogeny "; this hypothesis, while false, contains elements of truth ...
Most fish species with pharyngeal teeth do not have extendable pharyngeal jaws. A particularly notable exception is the highly mobile pharyngeal jaw of the moray eels.These are possibly a response to their inability to swallow as other fishes do by creating a negative pressure in the mouth, perhaps induced by their restricted environmental niche (burrows) or in the air in the intertidal zone. [10]
Branchial arches or gill arches are a series of paired bony/cartilaginous "loops" behind the throat (pharyngeal cavity) of fish, which support the fish gills. As chordates , all vertebrate embryos develop pharyngeal arches , though the eventual fate of these arches varies between taxa .
In the jawless fish, the first gill opening immediately behind the mouth is essentially similar to the other gill openings. With the evolution of the jaw in the early jawed vertebrates , this gill slit was caught between the forward gill-rod (now functioning as the jaw) and the next rod, the hyomandibular bone , supporting the jaw hinge and ...
The gill arches of bony fish typically have no septum, so that the gills alone project from the arch, supported by individual gill rays. Some species retain gill rakers . Though all but the most primitive bony fish lack a spiracle, the pseudobranch associated with it often remains, being located at the base of the operculum.