200 Elementary Zoology. 



separated abdomen ; they breathe by tracheae when breathing 

 organs are present. 



Allied probably to the last group of the Arachnids are three 

 little understood groups of Arthropods. The Linguatulida are 

 ento-parasites of a vermiform appearance, but of Arthropod 

 characters. Their appendages are represented by hooks. The 

 Tardigrada are rather more degraded, though free-living 

 organisms ; they have been confounded with the Rotifera. 

 They have short clawed limbs and no respiratory organs. The 

 third group is that of the Pyenogonida, sometimes placed among 

 the Crustacea ; they are marine and spider-like with a small 

 body and four pairs of sprawling legs. The abdomen is 

 rudimentary. 



IX. Chordata. 



The group Chordata does not only include the Vertebrata, 

 but also a number of varied organisms, formerly of doubtful 

 affinity, which are now held to be more or less distantly related 

 to the Vertebrata. All of these animals agree with each other 

 to differ from any of the Invertebrate group in the three fol- 

 lowing particulars : — 



1. There are either temporarily or permanently openings 

 from the pharynx to the exterior, the gill-slits. 



2. A dorsal rod of skeletal nature exists either temporarily 

 or permanently, and lies above the nervous system; this 

 notochord extends 'through a greater or less extent of the body, 

 and is hypoblastic in origin. 



3. The central nervous system is dorsal in position, and is 

 usually tubular in character. The Chordata, as thus defined, 

 may be divided as follows : — 



Class I. HEMICII0EDATA.—Snh-c\s.5s (a), Entero- 

 pneusta ; sub-class (6), Cephalodiscida ; sub-class (c), Rhabdo- 

 pluerida. 



Class 2. UROCHORDATA. 



Class 3. CEPHALOCHORDATA. 



Class 4. VERTEBRATA. 



