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The importance of comparative anatomy

Comparative anatomy explores and establishes the correspondences between body parts of organisms from different species. It builds the concepts of the living structures and thus must not be confused with morphology (the study of the forms and their variations) nor with Evo-Devo (the study of the relations between genetics of the development and evolution). Without comparative anatomy, naming and understanding what can be seen in organisms would be impossible. Then, neither descriptive embryology, causal embryology, phylogeny, palaeontology nor systematics could be conducted and the understanding of both biology and evolution of species would be dead-end streets.

Examining several organisms, we can establish functional correspondences (as between a wing of a fly and one of a pigeon) and correspondences of origin (as for a wing of a fly and one of a mosquito). These must not be confused, because they are different. Comparative anatomy is the study of both the functional adaptation, where the first type of correspondences plays the main role, and the phylogeny, where it is assumed that the second type of correspondences is important.

The study of macrostructures (in recent forms and in fossils) is far to be finished, even on familiar species such a perch (Percidae) or a catfish (Siluridae). The identification and the naming of the bones of the skull of silurids, and those of related forms as clariids, being still uncertain regarding the bones of a perch.

Wels catfish, Silurus glasne
Wels catfish, Silurus glanis, Siluridae.
Neurocranium of a silurid, dorsal view
Neurocranium of a clariid, dorsal view.
European Perch, Perca fluviatilis
European Perch, Perca fluviatilis, Percidae.

In muscles, a huge amount of possible new characters is going to be explored, such as intermuscular bones and ligaments (= transparent rods of the next image), and new functional adaptations are going to be deciphered (Chanet et al., 2004 [1]Chanet.ea.2004).

Structures intramusculaires chez un jeune turbot (Scophthalmus maximus, scophthalmidés)
Intermuscular structures in a young turbot (Scophthalmus maximus, Scophthalmidae)
(Photograph: F. Wagemans (Univ. Liège)).
Dissection of the oral cavity of a common sole (Solea solea, Soleidae).
(Images: B.. Chanet)
 Opercle, jaw and gills of the right side have been removed. Black arrows show thyroid islets (little red masses). The yellow arrow indicates an other endocrine gland : the ultimobranchial glands. Black squares indicate basibranchials, bones of the skeleton bearing the gills and forming "the tongue" of these animals. 
Comparative anatomy helps to propose homology hypotheses between different organs. In acanthomorphs, thyroid islets are scattered, more often associated with basibranchials (see above figures) but possibly present in other tissues as well (kidney, spleen, intestine ...). In sarcopterygians, the thyroid gland is a unique and encapsuled organ. By their anatomic position, their vascularisation and embryonic origin, these islets are similar to the thyroid gland and can be considered as homologous. This homology is corroborated by the cellular types present in these organs and by the hormones they produce. Both deliver T3 and T4, two hormones playing important roles in growth, development and metabolism. Moreover, these hormones are implied in the control of the metamorphosis when present (like in eels, amphibia and flatfishes).
Comparative anatomy helps to consider basibranchials as homologous to the different cartilages associated to the thyroid and present in the skeleton of the trachea (thyroid, cricoid and arytenoid cartilages) in mammals. These hypotheses of homology will be confirmed by the construction of a phylogeny, where these observations will be confrontated to the other inherited features in the studied organisms.

Today, comparative anatomy uses diverse techniques: classical dissection, examination of skeletons elements, radiographies and images obtained thanks to CT scanning or Magnetic Resonance Imaging (MRI) as on this alcohol preserved mackerel (Scomber scombrus, Scombridae).

MRI of a still in the jar common mackerel, Scomber scombrus, Scombridae
MRI of a still in the jar common mackerel (Scomber scombrus, Scombridae)
(Source : Unité d’Imagerie Médicale et Unité d’Anatomie Comparée (ENVN, Nantes)).