386 



ORDER VI. TUBULIBRANCHIATA. 



926. The Mollusks of this Order construct an irregularly 

 tubular shell ; which so much resembles that formed by certain 

 Annelida, as to be scarcely distinguishable from it. They are 

 very few in number. The Vermetus (Fig. 583) is the principal 

 genus of the group. This is remarkable for the close resemblance 



FIG. 583 VERMETUS. 



of its shell to that of the Serpula (Fig. 528) ; but, when perfect, 

 it may be generally distinguished by the regularly-spiral twist- 

 ing of its first-formed portion. Some species associate together 

 in large masses, so as even to form reefs ; whilst others attach 

 themselves to Coral, and lengthen their shells in proportion as 

 the Coral grows up around them. When the animal has quitted 

 the lower part of its tube, that it may keep itself at the surface, 

 it usually throws a partition across, in the manner of the 

 Nautilus ; and a tolerably regular series of such partitions is not 

 unfrequently found. In the Magilus, an allied genus of similar 

 habits, whose tube is sometimes lengthened in this manner to as 

 much as three feet, the first-formed part of the shell, instead of 

 being cut off, is sometimes completely filled up by the exudation 

 of solid matter. 



ORDER VII. SCUTIBRANCHIATA. 



927. This Order is also a small one, containing but two prin- 

 cipal genera, which do not differ widely from the Limpets, except 

 in the disposition of the gills. The shells are very open, without 

 an operculum, and the greater number are not in any degree 



