//. TYPES IN DETAIL IVITH THEIR SUBORDINATE GROUPS. 35 



Order i. Regularia Mouth and anus in the centre of 



their respective surfaces. 



X Arbacia, Toxopneustes. 



Order 2. Clypeastroidea Mouth central, anus eccentric, 



often near the outer margin. 



X Echinarachnius. 



Order 3. Spatangoidea Mouth and anus both eccentric. 



X Spatangus. 



Aside from these classes there are two wholly fossil groups, often 

 reckoned as classes, the Cystidea and the Blastoidea. These seem 

 allied to the Crinoids. The Cystidea possessed irregular calyx plates 

 and some of them were without arms. The Blastoidea were all arm- 

 less, but were regular in structure, with well-marked ambulacral areas. 



TYPE VII. PREVERTEBRATA, 



A group of apparently very dissimilar animals, all of which pos- 

 sess gill slits in the pharyngeal region, a dorsal nervous system and 

 a more or less well-defined endoskeletal rod, the notochord. They 

 are thus seen to be allied to the Vertebrates and undoubtedly repre- 

 sent the much-modified descendants of the transitional forms which 

 became the ancestors of that group. 

 Class 1. ENTEROPNETJSTA ...... Small, worm-like animals, with 



habits similar to Annelids, and 

 found between tide-marks in mud 

 flats. At the anterior end is a 

 conical and flexible proboscis with 

 which the animal pushes its way 

 through the mud; this is followed 

 by the " collar " at the upper ven- 

 tral edge of which is the mouth ; 

 below this is a respiratory region, 

 perforated by pairs of gill-slits 

 which communicate with the pha- 

 rynx. A small dorsal diverticulum 

 at the anterior end of the pharynx 

 represents the notochord. 



X Balanoglossus, Harrimama. 



