TEREDO. 155 



a mantle, which at first forms the shell and afterwards 

 the pallets and sheath ; the cilia, which invest most (if 

 not all) embryonic forms are absorbed, and a foot is 

 produced out of the firmer tissues of the body, and 

 substituted for the cilia; the eyes, mouth, palps, 

 stomach, intestine, liver, heart, gills, muscles, nerves, 

 reproductive and other organs come upon the stage 

 and play their several parts. " Instinct " does duty as 

 prompter. This, the inventive faculty of every creature 

 but man, provides for its necessities of food and de- 

 fence, and dictates the nature of its habits by an in- 

 scrutable kind of prescience, that is little less than 

 divine. Laurent, Lukis, and others have also noticed 

 the great activity of the fry in their intermediate state ; 

 and M. Kater observed them swimming freely about 

 the piles in the dykes of Holland, and after a while 

 attaching themselves to the wood. Like the oyster-fry, 

 they seem capable, to a certain extent, of selecting their 

 habitat, and they probably use their eyes for that pur- 

 pose ; but this can only be the case when the sea is 

 unusually calm, their puny force being quite un- 

 equal to contend with any agitation of the water. I 

 have just re-examined a piece of wood to which some of 

 the fry of T. navalis still adhere. Each is no bigger 

 than the smallest pin's head, and is enclosed in a pair of 

 somewhat oval, close-fitting, semimembranous, and yel- 

 lowish valves, the only opening in which serves as a 

 passage for the foot or point of attachment. It bears 

 some resemblance to a minute Cythere or crustacean of 

 the Entomostracan kind, as well as to the pupa or last 

 larval state of a Cirripede. The original or rudimentary 

 valves are persistent, and form the umbonal portion of 

 the perfect ones ; they are easily recognizable in young 

 specimens by their different shape, consistency, and co- 



