VERTEBRATA ACRANIA. 



135 



coasts, as, for example, the Mediterranean, and has been found 

 off our own shores. 



All the types hitherto described belong to the Invertebrata, or 

 animals devoid of backbone, while Amphioxus, Dog-fish, Frog, 

 Pigeon, and Rabbit, the remaining animals to be dealt with, are 

 examples of the Vertebrata, or backboned group, using the word 

 in a very broad sense, and by " backbone " understanding a firm 

 rod, not necessarily of bone, underlying and supporting the central 

 nervous system. To avoid confusion it is perhaps advisable to 

 drop the name Vertebrata as used in the broader sense, replacing 

 it by the term Chordata, and styling the Invertebrates Non- 

 Chordata for the sake of uniformity. All Chordate animals 

 possess, temporarily or permanently 



(1) A tubular central nervous system running along the dorsal 

 side of the body, and not perforated by the gut as in many higher 

 Non-Chordates (e.g., Earthworm, Leech, Crayfish). 



(2) A firm elastic rod, the notochord, underlying the central 

 nervous system for part or all of its length. 



(3) [Except in one instance] A laterally perforated pharynx, 

 serving in the lower forms as a respiratory organ, 



The Chordata are classified as follows : 



CHORDATA ( = VERTEBRATA in wider sense). 



I. Heinichorda. Small notochord in anterior 

 part of body. A small group of worm-like 

 forms. 



II. Urochorda. Small notochord in tail of 

 larva, generally absent in adult. A group 

 of degenerate forms, the Ascidians or Sea- 

 Squirts, which are usually fixed when 

 adult. 



III. Cephalochorda. Notochord extending from 

 one end of the body to the other. 

 Amphioxus. 



IV. Vertebrata (in narrower sense). Noto- 

 chord ends anteriorly below the middle of 

 the brain. A more or less complete ver- 

 tebral column ("backbone") in which 

 segmentation is always indicated, and 

 which usually encroaches more or less 

 upon the notochord. -Dogfish, Frog, Lizard, 

 Fowl, Rabbit. 



= VERTEBRATA 

 ACRANIA. 



Nobrain-case,limbs, 

 or jaws. 



= VERTEBRATA 



CRANIOTA. 

 A brain-case. 

 Limbs, when present 

 riot more than two 

 pairs. 



