tr>i; 



ZOOLOGY. 



caeca placed on either side; the anus in at the extremity of 

 the body. 



The blood is almost always red ; sometimes it is green, and 

 at others scarcely coloured ; it circulates in a complex system 

 of vessels, varying in different species; of these vessels M inn- 

 are contractile, and perform the 

 function of a heart ; others those 

 of arteries and veins. 



The respiration of these animals 

 is generally aerian, but sometimes 

 aquatic, and in this case it is per- 

 formed by means of external bran- 

 chiae, whose form and arrangement 

 vary much ; sometimes they re- 

 semble little trees or leaves, and 

 are fixed above the feet on each 

 side of the back, as in the areni- 

 cola (Fig. 47) ; at other times they 

 resemble bunches of feathers, and 

 unite in a corona around the extre- 

 mity of the body, an arrangement 

 of which we have an example in 

 the serpula (Fig. 442). 



585. Most of the annelides 

 live in the sea, and several construct 

 as dwellings a long tube, formed 

 either of calcareous matters secreted 

 by the skin (Fig. 442), or consisting 

 of sand and fragments of shells, agglutinated by means of a 

 gelatinous substance; several, as the arenicola, plunge deeply in 

 the sand (Fig. 47) ; others conceal themselves under stones ; 

 finally there are annelides, as the leech, which live in fresh 

 \\ at ITS ; so also does the nais, which more resembles the earth- 

 worm ; and these last, called by zoologists lumbrici, are land 

 animals. 



Fig. 442. A Group of 

 SerpuJse. 



CLASS OF THE EOTIFEEA. 



586. These beings, which have been often confounded with 

 the infusoria, are nevertheless quite distinct. Their structure 

 is very complex, and we owe the discovery of this fact to the 

 microscope, and to the profound researches of M. Ehrenberg, 

 of Berlin. Before his time they were thought to be animals 



