Finally, throughout geologic time, invertebrates have remained non-craniate creatures; that is, they never developed a cranium, nerve-chord brain, skull, or hard protective braincase ( unlike many vertebrates ).
12.
Aquatic craniates have gill slits, which are connected to muscles and nerves that pump water through the slits, engaging in both feeding and gas exchange ( as opposed to lancelets, whose pharyngeal slits are used only for suspension feeding ).
13.
The enigmatic " Yunnanozoon lividum " is considered to be the earliest hemichordate, possessing many of the characteristic chordate features and providing an anatomical link between invertebrates and chordates . " Haikouella lanceolata " is described to be the earliest craniate-like chordate.
14.
Olfaction is not a power that mammals or fish developed in preexisting nostrils, but rather the nose and related organs evolved around olfaction, and to say the brain in craniates is an outgrowth of the olfactory lobe at its nexus with the locomotory nervous system is entirely justified ontogenetically.
15.
This may suggest that lamprey eyes are indeed the most primitive " proper " craniate eyes extant . ( Hagfish, on the other hand, is something of an enigma; either their eyes are the basal chordate eyes, or they have lost the advanced eyes their ancestors possessed, as the hagfish ecological niche does not require vision.
16.
A "'craniate "'is a member of "'Craniata "'( sometimes "'Craniota "'), a proposed clade of chordate animals that contains the Myxini ( hagfish ), Petromyzontida ( including lampreys ), and the much more numerous Gnathostomata ( jawed vertebrates ) as living representatives.
17.
Over half of all living vertebrate species ( about 32, 000 species ) are fish ( non-tetrapod craniates ), a diverse set of lineages that inhabit all the world's aquatic ecosystems, from snow minnows ( Cypriniformes ) in Himalayan lakes at elevations over to flatfishes ( order Pleuronectiformes ) in the Challenger Deep, the deepest ocean trench at about.
18.
Lampreys ( already mentioned by Looie ) indeed have an eyeball that is moved by extraocular muscles . ( To be fair, lampreys have 5 extraocular muscles while most craniates-- from fish to mammals-- have 6; also, in contrast to most craniates, lamprey eyes are also " focused " by the extraocular muscles; AFAIK lampreys lack ciliary muscles in their eyes ).
19.
Lampreys ( already mentioned by Looie ) indeed have an eyeball that is moved by extraocular muscles . ( To be fair, lampreys have 5 extraocular muscles while most craniates-- from fish to mammals-- have 6; also, in contrast to most craniates, lamprey eyes are also " focused " by the extraocular muscles; AFAIK lampreys lack ciliary muscles in their eyes ).
