Online Encyclopedia

lower vertebrates and so to represent the invertebrate stock from which Vertebrates have been derived. Vertebrates, or as they are sometimes termed Chordates, are distinguished from other animals by several important features. The See also:

CHIEF (from Fr. chef, head, Lat. caput)

chief of these is the presence of an elastic See also:

rod, the notochord, which forms the See also:

LONGITUDINAL

longitudinal See also:

AXIS (Lat. for " axle ")

axis of the See also:

BODY

body, and which persists throughout See also:

LIFE

life in some of the lowest forms, but which appears only in the embryo of the higher forms, being replaced by the jointed backbone or vertebral See also:

COLUMN (Lat. columna)

column. A second feature is the development of outgrowths of the pharynx which unite with the skin of the See also:

NECK (O. Eng. hnecca; the word appears in many Teutonic languages; cf. Dutch ride, Ger. Nacken; in O. E. the common word was heals; cf. Ger. Hals)

neck and See also:

FORM (Lat. forma)

form a See also:

SERIES (a Latin word from serere, to join)

series of perforations leading to the exterior. These structures are the gill-slits, which in fishes are lined with vascular tufts, but which in terrestrial breathing animals appear only in the embryo. The third feature of importance is the position of structure of the central See also:

NERVOUS

nervous See also:

SYSTEM

system, which in all the Chordates lies dorsally to the alimentary See also:

canal and is formed by the sinking in of a longitudinal See also:

MEDIA

media dorsal groove. Of these structures the See also:

VERTEBRATA

Vertebrata or Craniata possess all three in a typical form; the Cephalochordata (see AMPInoxus) also possess them, but the notochord extends` throughout the whole length of the body to the. extreme tip of the snout; the Urochordata (see See also:

TUNICATA

TUNICATA) possess them in a larval See also:

CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition, but the notochord is See also:

PRESENT

present only in the tail, whilst in the adult the notochord disappears and the nervous system becomes profoundly modified; in the Hemichorda, the See also:

RESPIRATORY

respiratory See also:

ORGANS

organs very closely resemble gill-slits, and structures comparable with the notochord and the tubular dorsal nervous system are present. The Hemichorda include three orders, the See also:

PHORONIDEA

Phoronidea (q.v.), the See also:

PTEROBRANCHIA

Pterobranchia (q.v.) and the Enteropneusta (see See also:

BAr.ANOII GLOSSUS), but the relationship to the Chordata expressed in the designation Hemichordata cannot be regarded as more than an attractive theory with certain arguments in its favour. (P. C.

End of Article: HEMICHORDA, or HEMICHORDATA

Additional information and Comments

There are no comments yet for this article.

» Add information or comments to this article.

Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.

Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
HEMEROBAPTISTS

[next]
HEMICYCLE (Gr. iift-, half, and avahos, circle)