PROGRESSIVE GROWTH OF EMBRYO. 297 



head, antennae, eyes, and locomotive feet, wherewith it 

 swims about wherever it thinks proper ; whereas the 

 adult Terebella, the resident in the tube, has neither 

 head nor eyes, nor antennae ; and, as for its feet, they 

 are little better than hooks, wherewith it moves about 

 in the interior of its beautifully constructed tube. 



r 



The further progress of development is therefore 

 rather retrograde than otherwise, the worm becoming 

 deprived of privileges which it previously possessed. 

 Wonderful are the means whereby the ends of nature 

 are thus accomplished : the Terebellre, as such, must 

 always have remained fixed in the same locality, with- 

 out the power of distributing their species beyond 

 the spot where they have ensconced themselves ; but 

 as it is, the wandering larva roams from coast to 

 coast in search of a resting-place adapted to its pecu- 

 liar habits. 



By degrees, however, the erratic habits of our little 

 worms are put a stop to; they lose their ciliated 

 belts, and cease to swim, content to lead during the 

 remainder of their lives a sedentary existence. The 

 first indication of this change is the exudation of a 

 kind of mucus, which solidifies around their bodies, 

 and soon forms a tube open at both ends prehensile 

 booklets begin to make their appearance, supplanting 

 the functions of the swimming feet and the ten- 

 tacula of the Terebella begin to sprout from the fore- 

 head the eyes disappear, or rather, strange to say, 

 become replaced by others of a different kind the 

 gills or branchial tufts begin to sprout from the neck, 

 the number of segments goes on increasing, and the 

 tube-building Terebella, at last complete in all its 



o5 



