64 



INTRODUCTION TO ZOOLOGY. 



one of the marine worms, the last joint of the body gradually 

 extends, and increases to the size of the rest of the animal ; 

 and a separation is made by a narrowing of the preceding 

 joint, which at last divides. Previously to its 

 separation, however, the young one often shoots out 

 I a young one from its own last joint, in a similar 

 manner, and three generations have thus been seen 

 united." It is a curious circumstance, that the same 

 tail serves as the tail of successive individuals, and 

 seems thus to enjoy an exemption from the ordinary 

 laws of mortality. 



Respiration in the earth-worm is carried on by 

 means of pores and internal sacs, similar to those 

 of the leech. In the "lob- worm,"* or "lug of 

 fishermen (Fig. 39), a portion of the body is fur- 

 nished with little arborescent (tree-like) tufts, to 

 \ which the blood is conveyed, and there purified, by 

 'I coming into contact with the air diffused through 

 | the sea-water. 



In the next tribe of Annelids, a new modification 

 I of the respiratory organs is exhibited, one ad- 

 mirably adapted to their peculiar habitats and 

 modes of life. All the individuals of this assem- 

 blage dwell in tubes, consisting either of calcareous 

 matter, secreted from their own bodies, or, as in 

 the Terebella, of particles of sand and gravel 

 agglutinated together to serve as a habitation. 

 Under these altered circumstances, the only place 

 to which the vivifying principle of the sea-water 

 could freely have access, would be that adjacent 

 to the exterior orifice of the tubes; and here, 

 accordingly, we find the respiratory apparatus 

 arranged, often extremely graceful in its form, and 

 enriched with brilliant colouring. The small con- 

 torted tubes which encrust, in so fantastic a man- 

 rig. 39. ner, the old bottles or dead shells dredged up from 

 COLA 'any of our bays, form an example of this class. 

 They are the dwellings of one of these sedentary worms, 



* This was formerly classed with the earth-worm, under the name of 

 Lunibricus marinus ; but, from its difference of structure, it is now 

 referred to a different order (^Doraibranchiata^ and bears the name 

 Arenicola piscatorum. 



