ARCHICG@LOMAT-. 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 
ie in most cases arises by pouches from the hypo- 
ast. 
Sus-PoyLum 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 Folyzoa. 
Sup-Puytum II.—EcHInopERMaAtA.—As¢erias is a fair 
representative of this large sub-phylum. They all show 
plano-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 ccelom into a water-vascular (or ambulacral) 
system. The larve show the archimeric segmentation of the 
Archicelomata. The peculiar axial symmetry is usually 
supposed to be due to a sedentary or fixed habit in the 
past. Like most Archicalomata, they are well represented 
in early epochs. 
There are five classes of the Echinodermata :— 
Crass I.—Asteroidea, of which Asferias is a type. 
Ciass II. — Ophiurordea (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. ’ 
