|Homologous structures in chordates
Lancelets and lamprey larva may resemble the ancestor of all vertebrates. As a motile creature, such an ancestor seems to have been the starting point for a series of radiations, resulting in the extant groups of vertebrates and many other extinct groups. Some new features appeared during the course of vertebrate evolution, but for the most part the diverse features of mammals, reptiles, fish, birds, etc., are derived from structures seen in the lancelet and lamprey larva. In the following sections you will review the evolution of some of these structures, tracing the homologies of the chordate body plan. Refer to the chordate pedigree when necessary, since you will be tracing the evolution of structures, rather than the evolution of groups per se. Groups that diverged at different times, such as sharks and bony fishes, will be used to illustrate some intermediate stages in the evolution of structures. Keep in mind, though, that each group of vertebrates is adaptive in its own right, so that while these groups may retain some "primitive" features, the animals themselves may not be primitive at all - but are the products of their own long evolutionary pedigrees.
As you review the homologies presented here, be particularly aware of structures whose functions have become modified through time, and of the characteristics of the original structures that may have lent them the possibility of a change in function (such as their location or form).
(A) Structures homologous with the dorsal nerve tube
The spinal cord retains the form of a hollow nerve tube throughout life, with a hollow core and segmentally arranged spinal nerves. The brain also retains aspects of this form. The walls of this tube later becomes thick, as neurons form the mass of the brain proper. But its tubular, hollow nature is retained. Examine the sheep brain, and find the ventricles, which are the remnants of the inside of the tube.
Beyond its origin as part of the dorsal nerve tube, the evolution of the brain was probably tied with the evolution of the three senses of the head: olfaction, vision, and hearing/equilibrium. The organs for each of these senses develop embryologically in close association with the brain. The nerve-bearing parts of the eyes develop as outgrowths of the brain itself, and the olfactory and auditory organs develop as vesicles (spheres of cells) near the brain. The developing brain differentiates into three regions, each associated with one of the senses: the forebrain (prosencephalon), associated with olfaction, the midbrain (mesencephalon), associated with vision, and the hindbrain (rhombencephalon), associated with audition. A thickened layer of gray matter (neurons with no myelin sheath) develops on the roof of each region, and serves to process the information from sense organs. Development proceeds in this manner in all vertebrates, and the brains of many vertebrates (especially fishes) retain this organization throughout life. Examine the models of vertebrate brains, and find the three regions in each type of brain. One of the major changes in vertebrate brains in tetrapods is the differentiation and growth of the forebrain, which, as the cerebral hemispheres, dominates the size and function of the brain.
(B) Homologies in the chordate skull
The lancelet has no skull; in fact it has no real brain, since the notochord extends all the way to its snout. In vertebrates, however, the skull protects the brain and sense organs and serves as a fulcrum for the body muscles, pectoral girdle, and jaws. Where did the skull come from? We don't know for sure, and must rely again on embryological evidence. From the beginning, though, it is very complex amalgam of bones (or cartilage) originating in three parts of the skeleton. The brain case proper makes up the base of the skull and the housing of the sense organs. Dermal bones, derived from flat, scale-like bones in the skin, make up the roof of the skull. The gill bars make up a prominent feature of the skeleton near the head in fishes, and contribute to the skull of tetrapods as well.
Examine the skull of a shark. It consists solely of brain case. Notice at the anterior end the nasal capsules and near the posterior end the ear (otic) capsules. Much of the brain case develops embryologically from cartilaginous capsules surrounding the sense organs: the olfactory, eye, and ear capsules. The eye capsules in the shark remain only as cartilages surrounding the orbit. These capsules, plus a few other sources of cartilage, grow together to make the floor of the brain and the housings for sense organs.
To see the contribution of dermal bones to the skull, examine the head of Amia, the bowfin. Note the armor-plated appearance of its head (and, for later reference, the teeth on the bones at the edge of the mouth). These plates are dermal bones, which develop within membranes in the deep layer of the skin. They are homologous with scales, and even resemble large scales in the bowfin. These dermal bones cover the roof of the skull, and are evident in bony fishes and tetrapods as the flat bones of the front, top, and rear of the skull. By examining the skull of Amia from behind and below, you can clearly distinguish the contributions of the braincase and dermal bones to the skull.
The brain case and dermal elements (plus the derivatives of gill supports) make up the skull of all other vertebrates. In the more advanced groups, like the modern bony fishes and tetrapods, the trend has been towards a fusion of bones and reduction in the overall mass of the skull. Observe the skulls of bony fish, reptiles, birds and mammals, and try to distinguish dermal versus brain case elements.