and the Embryolog-ij of Autolytus curnvtus. 407 



Fig. 19. — Head of a male somewhat more advaiiced 

 than that of fig. 15 ; there are two eyes, and tlie tentacles 

 are forking. 



Fig. 20. — Head of a male ready to separate ; the 

 middle tentacle a, as well as the long cirri c", are car- 

 ried tightly curled up. 



Plate X. 



Development of the Parent Stock. 



Fig. 1. — A full-grown parent stock previous to the for- 

 mation of male and female individuals, seen from above. 



As the embryos which develop from the eggs are the 

 parent stocks from which the males and females are sepa- 

 rated, the letters are all capitals, and correspond to those 

 of fig. 9, PI. IX. 



Fig. 2. — A young parent stock of Autolytus just hav- 

 ing made its escape from the sac. 



Fig. 3. — The digestive cavity has become more insu- 

 lated, the anterior part shows a tendency to differentia- 

 tion. The body has lost somewhat its triangular outline. 



Fig. 4. — The alimentary canal is divided into three 

 distinct regions, and the body has assumed a more elon- 

 gated form than in the preceding figures. Sinuosities 

 indicating the rings are clearly seen. 



Fig. 5. — The rings have become very distinctly marked, 

 the head is more distinct from the rest of the body. 



Fig. 6. — Fig. 5 seen in profile. 



Fig. 7. — Head of a young parent stock of Autolytus 

 somewhat more advanced than in preceding figures. 



Fig. 8. — The lateral tentacles A', A" extend beyond the 

 body. 



Fig. 9. — The tentacles of the head are all equally ad- 

 vanced ; bristles appear at three of the rings ; the caudal 

 stiles are slight swellings on each side of the anal ring, E. 



Fig. 10. — Although not quite as advanced as fig. 9, 

 has one more ring. 



