Apical 

 plate 



Fig. 18.1. Phylum Hemichordata. A, general features of a typical balanoglossid, SaccoiJlossiis 

 kowalewskii. B, general features of the tornaria larva, a free-swimming developmental stage 

 in the life cvcles of many balanoglossids. 



are invertebrates; but their affinities with the phylum Chordata are so evident 

 that we have reserved them for discussion with the chordates. 



The Hemichordata constitute a minor group of invertebrates, exclusively 

 marine, which are either sedentary, burrowing, worm-like forms or highly 

 modified, sometimes colonial animals attached to the substrate or floating in 

 the sea. In contrast, the Chordata are highly diversified, and they are in fact 

 the most widely distributed group of animals, with the possible exception of 

 the arthropods. They are found in the ocean at all depths, and on land 

 from rain forest to desert habitats, and from the tropics to the polar regions, 

 where representatives of the warm-blooded birds and mammals occur. 

 Chordates range in size from minute forms to whales, which are the largest 

 of existing animals. In diversity of habitat and structure, only the arthropods, 

 again, rival the chordates. The distribution of arthropods on land is limited, 

 however, by their inability to remain active at low temperatures; and even 

 large arthropods, such as some crustaceans, are pygmies in comparison with 

 the largest chordates. 



The phylum Chordata comprises two large subdivisions; the division Acrani- 

 aia and the division Craniata or Vertebrata. The Acraniata afe tho.se chor- 

 dates without skulls and without vertebral columns, in which the notochord is 



539 



