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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]
Human vestigiality is related to human evolution, and includes a variety of characters occurring in the human species. Many examples of these are vestigial in other primates and related animals, whereas other examples are still highly developed. The human caecum is vestigial, as often is the case in omnivores, being reduced to a single chamber ...
It is postulated that this is how pharyngeal slits first assisted in filter-feeding, and later, with the addition of gills along their walls, aided in respiration of aquatic chordates. [1] These repeated segments are controlled by similar developmental mechanisms. Some hemichordate species can have as many as 200 gill slits. [2]
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 .
Cavefish or cave fish is a generic term for fresh and brackish water fish adapted to life in caves and other underground habitats. Related terms are subterranean fish, troglomorphic fish, troglobitic fish, stygobitic fish, phreatic fish, and hypogean fish.
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 ...
A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are obligate ram ventilators and would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species. [3] The true gill slits in embryonic fish develop into fish gills.