50 



OUTLINES OP NATURAL HISTORY. 



side of the mouth. To the gullet are attached certain 

 muscular fibres {m), wHch are fixed below to the bottom 

 of the cell ; and it is by the shortening of these that the 

 animal pulls itself into the cell. 



Between the mouth and vent is placed a little nervous 

 mass, which constitutes the central portion of the nervous 

 system (n). There is no heart, nor any blood-vessels ; nor 

 are there any distinct breathing-organs. The fluid which 

 fills the cell consists partly of water and partly of the 

 products of digestion, and it is to be regarded as corre- 

 sponding with the blood of higher animals. It is kept in 

 movement by means of little vibrating hair-like processes 

 which cover the membranous lining of the cell ; and it is 

 exposed to the action of the oxygen contained in the 

 water as it circulates through the crown of " tentacles," 

 these latter organs being hollow. 



The broad-leaved Sea-mat is of common occurrence in 

 the seas of Britain, in a few fathoms of water. It attains 

 a height of about four inches, and has a wood-brown 

 colour. When fresh, it is stated to possess a peculiar 

 odour (not always present), which has been variously 

 compared to the scent of oranges, violets, or a combina- 

 tion of roses and geraniums, but which others consider 

 strong and disagreeable (Johnston). 



Recapitulation op Essential Characters. — Ani- 

 mal compound, consisting of numerous, nearly inde- 

 pendent beings, each of which is enclosed in a separate 

 chamber or cell. (This last character is not absolutely 

 universal.) Each member of the compound growth has 

 a mouth surrounded by tubular tentacles, a complete 

 alimentary canal opening by a distinct vent, and a 

 nervous system consisting of a little nerve-centre placed 

 on one side of the mouth. There is no heart, nor are 

 there definite breathing-organs. Almost universally the 

 colony is attached to some foreign object. These char- 

 acters distinguish the Polyzoa as a whole. 



