ARCHICCELOMATA. 171 



In their development there is, in the majority of cases, an 

 equal segmentation, a blastula, gastrula and a pelagic larva ; 

 in fact, a typically larval development throughout. The 

 coelom in most cases arises by pouches from the hypo- 

 blast 



Sub-Phylum I. — Archichorda. — Balanoglossus is the 

 burrowing vermiform representative of this sub-phylum, but 

 there are also sedentary relatives. They have special interest 

 as they appear to be allied to the ancestors of the metamer- 

 ically segmented Chordata. Thus the sub-phylum shows a 

 dorsal nervous system, pharyngeal clefts and chordoid por- 

 tions of the pharynx. This relationship will be mentioned 

 in the Chordata. 



Cephalodiscus is a small deep-sea form which lives in 

 sedentary communities. There is only a single pair of 

 pharyngeal clefts and two pleurochords. Others approxi- 

 mate in habits to the Polyzoa. 



Sub-Phylum II. — Echinodermata. — Asterias is a fair 

 representative of this large sub-phylum. They all show 

 piano-symmetric larvae, which go through a metamorphosis 

 into the adult. Their special features are the great develop- 

 ment of a mesodermic calcareous skeleton and a modification 

 of part of the coelom into a water-vascular (or ambulacral) 

 system. The larvae show the archimeric segmentation of the 

 Archiccelomata. The peculiar axial symmetry is usually 

 supposed to be due to a sedentary or fixed habit in the 

 past. Like most Archiccelomata, they are well represented 

 in early epochs. 



There are five classes of the Echinodermata : — 



Class I. — Asieroidea, of which Asterias is a type. 



Class II. — Ophiuroidea (the brittlestars). — These are five-rayed, 

 but the arms are almost entirely filled by the enlarged ossicles 

 and are sharply distinguished from the central disc. The greater 

 flexibility of the arms enables them to be largely used as motor 

 organs and the tube-feet are correspondingly reduced. There is no 

 anus and the madreporite appears to have become shifted to the 

 ventral surface. They differ in several other points from the 

 Asteroidea. 



