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The tooth shell, or Dentalium, is the lowest of its class, and its 

 life history is one of much interest. For the following facts we 

 are indebted to the memoir of Lacaze-Duthiers. The sexes are 

 distinct. It breeds from the beginning of August until the middle 

 of September. After fertilization by the spermatic particles, which 

 Lacaze-Duthiers saw penetrating into the egg, the egg undergoes 

 complete segmentation (A). At the end of this time the embryo 

 swims about by means of tufts of fine cilia (Fig. 129, B), and a 

 pencil of large cilia in front. It then lengthens and is provided 

 with seven bands of cilia, and the larva is remarkably worm-like 



Fig. 129. 



(C). When two days old the mantle secretes a small shell (a) at 

 the end of the body. The ciliated bands now approach and form 

 a swollen ring, or ciliated crown, the velum, as in fig. 129, D, z. 

 At this time the shell is median, unpaired and situated on the back 

 of the larva. The lobes of the foot next arise. Fig. 129, E, re- 

 presents the young Dentalium, after leaving the larval state, and 

 when thirty-five days old. The three-lobed foot protrudes from 

 the shell now enclosing the animal, the rudimentary tentacles 

 (E, d) are visible, as well as the suboesophageal nerve-ganglia 

 (E, j) and the digestive canal (E, /, /') and liver (/). After this, 

 the change into the mature form is slight. 



