ONTOGENY OF THE ANNULUS VENTRALIS. 



135 



showed very definite tuberosities separate from the hood and the 

 transverse fold and the tuberosity upon one side of the animal 

 extended over the median line, in carrying out the asymmetry of 

 the adult. The general proportions of the various folds and 

 thickenings of the shell now approximated the adult condition. 



The receptacle had not materially changed from the previous 

 stage, Fig. 21, but its invaginated shell walls were greatly thick- 

 ened and laminated, Fig. 24, enlarged like Fig. 21. It would 

 appear that with the dropping of the bottom of the invaginated 



cx^ 



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 <J*^~: *- - -Ib>~ -= - 



^^^S^^x 



v x\\ x >t->-i :-->5>$ov v 



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FIG. 24. 



groove away from the surface its sides have closed in to form a 

 narrow crevice which comes to the surface as a suture line. Then 

 by the bending of the bottom of the invagination more than its 

 surface suture line these crevices are made into curved oblique 

 planes. The hood and the transverse fold and the posterior 

 opposing fold are intimately associated with the bending of the 

 original groove to one side and then the sinking of that lateral 

 bend away from the surface and diagonally forward. 



The last young to be considered, the larva, 53 mm. in length 



