HALF AN HOUR WITH SEA-WORMS. 77 



exhibit the delicate sensibility we have noticed as 

 peculiar to the serpula. Occasionally they burrow 

 in the sand, for the purpose of inserting their tubes. 

 Others seem to prefer absolute freedom. The ex- 

 quisite form and graceful motions of the gills or 

 branchiae of the Sabella have long been favourite 

 topics for naturalists to descant upon. The Terebella 

 is another sea-worm allied to the latter in many 

 respects. It puts one in mind of the little caddis- 

 worms to be met with in our ponds and ditches, more 

 than anything else. You can hardly fail to meet with 

 it if you turn over a heap of seaweeds lying between 

 tides. The tubes are about the thickness of an 

 ordinary tobacco-pipe, and are composed of sand, 

 fragments of shells, &c., agglutinated together into 

 a flexible tube. The mouth of the tube is fringed 

 with a number of smaller hair-like tubes of a similar 

 construction (Fig. 34). This worm is remarkable 

 on account of the transformations through which its 

 young pass. They appear first as embryos, furnished 

 with cilia for swimming, and possessing a mouth 

 and anus and alimentary canal. By-and-by the 

 embryo gradually lengthens, the cilia become con- 

 fined to a band behind the head and another near 

 the tail. New segments are added one after the 

 other, the tubercles and setaa developing in the same 

 order, until at length a free-swimming Annelide 

 ensues. This remarkable development of the egg of 

 one animal to pass through the exact stage of 

 another, is frequently repeated in the lower forms 



