54 ZOOLOGY 



there are embedded bristles similar to those found in the skin 

 of most Annelida. 



Another phylum which deserves a brief mention is that 

 of the Polyzoa. These are small animals of almost micro- 

 scopic size, which like the Ccelenterata form colonies by budding. 

 They are however widely different from Ccelenterata, to the 

 colonies of which their colonies present some external resem- 

 blance. Each individual of a Polyzoan colony has, it is true, 

 a mouth surrounded by tentacles, but it possesses a complete 

 alimentary canal with mouth and anus surrounded by a well- 

 developed secondary body-cavity. The body of each individual 

 of the colony consists of two portions : a part in which the 



mouth and anus are situated, 

 in which the skin is soft, and 

 alower part in which the skin 

 secretes a hard cuticle. The 

 soft part is called the poly- 

 pide, the hard part the zoce- 

 cium. The polypide can be 

 withdrawn within the zoce- 

 cium by being turned outside 

 FIG. 24. Diagram showing two in when the animal is alarmed, 

 individuals of Polyzoon colony The masses of zooecia form 

 A 9 with extended, and B, with , r , rl 



retracted, polypide; a, knus; dellcate lace-like encrust* 

 o, mouth ; c, coelom. tions on the inner side of old 



shells, &c. The remains of 



the zooecia of extinct Polyzoa are amongst the commonest of 

 fossils. 



We now turn to a phylum which includes much more 

 familiar animals than the Polyzoa. The name of this phylum 

 is the Echinodermata, and the animals included in it are the 

 star-fish, sea-urchins, and brittle-stars of our coasts. Two 

 other less familiar groups likewise belong to the Echinodermata, 

 viz. the delicate feather-stars and the sausage-shaped sea- 

 cucumbers. All are inhabitants of the sea, and all are 

 characterised by having their organs arranged in radiate 

 symmetry around the mouth. By radiate symmetry is 

 meant that, instead of the organs being in pairs, as they are 

 in ourselves and in most of the phyla which we have here- 

 tofore discussed, they are arranged in Echinodermata like the 



