The Protochordata or Prochorda are simpler forms of the chordate animals in which the notochord is wholly or partially formed and the vertebral column and skull are lacking (hence Acrania). This division includes a heterogeneous (polyphyletic) group of animals of the phylum Chordata, most of which are fairly related to the vertebrates but none possesses [...]
In certain animals a structural condition occurs in which all or most of the paired parts or structures are repeated at regular intervals, along the antero-posterior axis of the body. The body of such animals is made up of a longitudinal series of elements, in each of which all or most of the systems of [...]
The Chordata include animals that can be grouped into two major subdivisions (subphyla) the Vertebrata (Craniata) and Protochordata (Acraniata). The vertebrates are the best known and most numerous representatives of this phylum, exceeded in number of species only by arthropods and molluscs, and widely distributed in all types of habitats. The other chordates, the Acraniata, [...]
The Chordata are animals characterized by the presence of a notochord, a dorsal gelatinous stiffening rod, present during some stage in development, tubular nerve cord, situated dorsal to the notochord and pharyngeal gill-slits.
