64 



INTKODUCTION TO ZOOLOGY. 



one of the mai'ine worms, the last joint of the body gradually 

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

 and a seoaration is made by a narrowing of the preceding 

 joint, which at last divides. Previously to its 

 separation, however, the young one often shoots out 

 a young one from its o^vn 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-woim 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 Avith little arborescent (tree-like) tufts, to 

 which the blood is conveyed, and there purified, by 

 coming into contact with the air diffused through 

 the sea-water. 



In the next tribe of Annelids, a new modification 

 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 Tercbella, of particles of sand and gravel 

 agglutinated together to serve as a habitation. 

 Under those altered cu'curastances, the only place 

 to which the vivif)'iug principle of the sea-water 

 could freely have access, would be that adjacent 

 to the exterior orifice of the tubes; and here, 

 accordingly, Ave find the respiratory apparatus 

 arranged, often extremely graceful in its form, and 

 enriched wdth brilliant colouring. The small con- 

 torted tubes which encrust, in so fantastic a man- 

 ner, the old bottles or dead shells dredged up from 

 any of our bays, form an example of this class, 

 the dwellings of one of these sedentary wonns, 



Fig. 39. 

 Arenicola 



They 



are 



* Tliis was formerly classed with the eaj-th-worm, under the name of 

 Lumhricus marinus ; but, from its difTerence of structure, it Ls now 

 referred to a dilferent order {Dorsibronchiata), aud bears the namo 

 Areiiicola piscalurum. 



