Editing Chondrichthyes (section)

==Anatomy==
{{See also|Cartilaginous versus bony fishes}}

===Skeleton===
The skeleton is cartilaginous. The [[notochord]] is gradually replaced by a vertebral column during development, except in [[Holocephali]], where the notochord stays intact. In some deepwater sharks, the column is reduced.<ref>{{Cite book|url=https://books.google.com/books?id=cxxSN4YA2i8C&dq=Notochord:+Chimaeroids+%22Some+deepwater+squaloid,+hexanchoid,+and+lamnoid+sharks+%22&pg=PA23|title=Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date|first1=Leonard J. V.|last1=Compagno|first2=Food and Agriculture Organization of the United|last2=Nations|date=20 November 2001|publisher=Food & Agriculture Org.|isbn=9789251045435 |via=Google Books}}</ref>

As they do not have [[bone marrow]], [[red blood cell]]s are produced in the [[spleen]] and the epigonal organ (special tissue around the [[gonad]]s, which is also thought to play a role in the immune system). They are also produced in the [[Leydig's organ]], which is only found in certain cartilaginous fishes. The subclass [[Holocephali]], which is a very specialized group, lacks both the Leydig's and epigonal organs.

===Appendages===
Apart from [[electric ray]]s, which have a thick and flabby body, with soft, loose skin, chondrichthyans have tough skin covered with dermal teeth (again, Holocephali is an exception, as the teeth are lost in adults, only kept on the clasping organ seen on the caudal ventral surface of the male), also called [[placoid scale]]s (or ''dermal denticles''), making it feel like sandpaper. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other.

Originally, the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and puboischiadic bars evolved. In [[Batoidea|rays]], the pectoral fins are connected to the head and are very flexible.

One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion.<ref>{{cite journal |first1=C. D. |last1=Wilga |first2=G. V. |last2=Lauder |title=Function of the heterocercal tail in sharks: quantitative wake dynamics during steady horizontal swimming and vertical maneuvering |journal=[[Journal of Experimental Biology]] |volume=205 |issue=16 |pages=2365–2374 |year=2002 |doi=10.1242/jeb.205.16.2365 |url=http://jeb.biologists.org/content/205/16/2365.short |pmid=12124362}}</ref>

===Body covering===
Chondrichthyans have tooth-like scales called [[dermal denticle]]s or placoid scales. Denticles usually provide protection, and in most cases, streamlining. Mucous glands exist in some species, as well.

It is assumed that their oral teeth evolved from dermal denticles that migrated into the mouth, but it could be the other way around, as the [[Teleostei|teleost]] bony fish ''[[Denticeps clupeoides]]'' has most of its head covered by dermal teeth (as does, probably, ''[[Atherion elymus]]'', another bony fish). This is most likely a secondary evolved characteristic, which means there is not necessarily a connection between the teeth and the original dermal scales.

The old [[placoderms]] did not have teeth at all, but had sharp bony plates in their mouth. Thus, it is unknown whether the dermal or oral teeth evolved first. It has even been suggested{{by whom|date=August 2018}} that the original bony plates of ''all'' vertebrates are now gone and that the present scales are just modified teeth, even if both the teeth and body armor had a common origin a long time ago. However, there is currently no evidence of this.

===Respiratory system===
All chondrichthyans breathe through five to seven pairs of [[gill]]s, depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills, whilst demersal species can actively pump water in through their [[Spiracle (vertebrates)|spiracles]] and out through their gills. However, this is only a general rule and many species differ.

A spiracle is a small hole found behind each eye. These can be tiny and circular, such as found on the nurse shark (''Ginglymostoma cirratum''), to extended and slit-like, such as found on the wobbegongs (Orectolobidae). Many larger, pelagic species, such as the mackerel sharks (Lamnidae) and the thresher sharks (Alopiidae), no longer possess them.

===Nervous system===
[[File:Skate Brain Regions.png|thumb|Regions of a Chondrichthyes brain colored and labeled on dissected skate. The [[Anatomical terms of location#Cranial and caudal|rostral]]{{Broken anchor|date=2025-03-24|bot=User:Cewbot/log/20201008/configuration|target_link=Anatomical terms of location#Cranial and caudal|reason= The anchor (Cranial and caudal) [[Special:Diff/1175671216|has been deleted]].|diff_id=1175671216}} end of the skate is to the right.]]
In chondrichthyans, the nervous system is composed of a small brain, 8–10 pairs of cranial nerves, and a spinal cord with spinal nerves.<ref>{{Cite journal|last=Collin|first=Shaun P.|date=2012|title=The Neuroecology of Cartilaginous Fishes: Sensory Strategies for Survival|journal=Brain, Behavior and Evolution|language=en|volume=80|issue=2|pages=80–96|doi=10.1159/000339870|pmid=22986825|s2cid=207717002|issn=1421-9743}}</ref> They have several sensory organs which provide information to be processed. [[Ampullae of Lorenzini]] are a network of small jelly filled pores called [[Electroreception|electroreceptors]] which help the fish sense electric fields in water. This aids in finding prey, navigation, and sensing temperature. The [[lateral line]] system has modified epithelial cells located externally which sense motion, vibration, and pressure in the water around them. Most species have large well-developed eyes. Also, they have very powerful nostrils and [[Olfactory bulb|olfactory]] organs. Their inner ears consist of 3 large [[semicircular canals]] which aid in balance and orientation. Their sound detecting apparatus has limited range and is typically more powerful at lower frequencies. Some species have [[Electric organ (biology)|electric organs]] which can be used for defense and predation. They have relatively simple brains with the forebrain not greatly enlarged. The structure and formation of myelin in their nervous systems are nearly identical to that of tetrapods, which has led evolutionary biologists to believe that Chondrichthyes were a cornerstone group in the evolutionary timeline of myelin development.<ref>{{Cite journal|last=de Bellard|first=Maria Elena|date=2016-06-15|title=Myelin in cartilaginous fish|journal=Brain Research|volume=1641|issue=Pt A|pages=34–42|doi=10.1016/j.brainres.2016.01.013|issn=0006-8993|pmc=4909530|pmid=26776480}}</ref>

===Immune system===
Like all other jawed vertebrates, members of Chondrichthyes have an [[adaptive immune system#Evolution|adaptive immune system]].<ref name=ref1>{{cite journal |last1=Flajnik |first1=M. F. |first2=M. |last2=Kasahara |title=Origin and evolution of the adaptive immune system: genetic events and selective pressures |journal=[[Nature Reviews Genetics]] |volume=11 |issue=1 |pages=47–59 |year=2009 |doi=10.1038/nrg2703 |pmid=19997068 |pmc=3805090}}</ref>