242 



niGH SCHOOL ZOOLOGY. 



CHAPTER IX. 



The Remaining Invertebrate Sub-Kingdoms. 



1. Four sub-kingdoms, tlie Echinodermata, Coelenterata, 

 Porifera, and Protozoa remain to be referred to ; with the ex- 

 ception of tlie last, however, they have but few fresh-water 

 representatives. In place of the bilateral symmetry of the fore- 

 going sub-kingdoms, a radial symmetry is often more notice- 

 able. On this account the groups in question were at one time 

 known as the.*' Radiata," but it must be understood that bi- 

 lateral symmetry may co-exist with the radial. 



2. The Echinoderms, exclusively a marine group, receive their 



name from the general 

 presence of an exoskele- 

 ton formed of more or 

 less regular calcareous 

 plates in the skin (Fig. 

 1 70), which cai-ry pro- 

 tecting spines. As a rule 

 the body is formed of 

 five similar rays or " an- 

 timeres " grouj)ed roiuid 



Fig. 170.— Echiims esculenfus. 1- a central axis. The in- 



Half of the shell is stripped of the spines, show- . , • • ,,enoll v rnm- 



ing the double rows of imperforate plates, a ; and teStUie IS USUaUy COm- 

 of those perforated by the tube-fee^-&. ^j^^.^ ^^^ contained in a 



spacious coelom, from which there is separated off during 

 development a system of blood-vessels, and also a characteristic 

 system of water-vessels. The chief stems of the latter answer 



