Animals exhibiting characters of two adjacent taxonomic groups are called connecting links. They afford vey good evidences of organic evolution. The following are the important connecting links:
Peripatus is a living connecting link between arthropods and annelids. Its arthropods characters are claws, jaws, haemocoel, tracheae and dorsal tubular heart. The annelidan characters are continuous muscle layers in the body wall, unjoined legs and nephridia.
Neopilina is a connecting link between annelids and molluscans. It is the only segmented mollusc. It has a cup shaped shell like that of limpet. Its visceral mass is divided into five segments, each with a pair of shell muscles, gills, auricles, and nephidia.
Protopterus (Lung fish) is a connecting link between fishes and amphibians.Like other fish, they have paired fins, dermal scales, gills, segmented trunk, tail muscles, and ear represented by internal ear only. They resemble amphibians in having internal nostrils, a lung and the heart with imperfectly divided auricle.
Chimaera is a connecting link between cartilaginous and bony fishes.
It resembles cartilaginous fishes in the following characters: cartilaginous skeleton, ventral mouth, two dorsal fins, clapers, placoid scales in the young.
It resembles bony fishes in the following characters: small mouth with fleshy lips, tooth plates joined to the jaws, four pairs of gills, absence of cloaca, separate anus and urinogenital aperture.
Balanoglossus is a connecting link between invertebrates and chordates. It is a protochordate.
It is a chordate because it contains a notochord, tubular nerve cord and gill slits. The invertebrates characters include the phosphogens and the larva.
Egg laying Mammals are the connecting link between reptiles and mammals. They are definitely mammals because they have mammalian characters like mammary glands, hair, diaphragm, single aortic arch etc. They resemble reptiles in having a large coracoid in the pectoral girdle and in laying eggs with yolk and shell. Eg: Duck-billed platypus or Ornithorhynchus.