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embryo becomes covered with cilia, most of which are later lost, although some persist, 

 forming a pre-oral and post-oral ciliated ring and a post-anal ciliated tuft. There is 

 also a tuft of cilia in the center of the flattened area. 



At an early stage a shell-gland is de\eloped from the ectotlerni of the dorsal part 

 (if the body, and this soon secretes a thin integument which is the rudiment of the 

 shell. At first the shell is single, but as it begins to extend around the body a median 

 dorsal line appears which separates the shell into two lateral valves; this corresponds 

 to the hinge line of the adult. 



Fig. 115. Photomicrograph of cross section of female Teredo imvalis in the region of the caecum, 

 showing marked development of the ovaries at the reproductive season. The small dark objects are 

 individual ova. 



Even after the shell has grown well around the body, the broad, flat pre-oral 

 portion with its ciliary circlet protrudes from between the \alves as a velum or swim- 

 ming organ. The larva is now known as a veliger, so named from this characteristic 

 organ, and at this stage it escapes from the brood pouch, is passed out through the 

 anal siphon of the parent, and must henceforth shift for itself. 



The velum forms a relatively powerful organ of locomotion, and the swimming 

 movements of the larval teredos as observed in an aquarium are quite vigorous. But 

 such minute organisms must be almost completely at the mercy of tides and currents. 



