ONTOGENY OF THE ANNULUS VENTRALIS. I 29 



Fig. ii. Iii C. Clarkii the groove has closed, leaving a suture, 

 represented by the black line, above an internal cavity, repre- 

 sented in white and this is bounded on the sides and bottom by 

 the infolded cuticle, represented in dotted lines. At the posterior 

 end the groove passes around the edge of the annulus and seems 



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FIG. 12. FIG. 13. 



to be still open there. At the front end the groove pushed for- 

 ward as a blind growth beneath the surface. Thus in C. Clarkii 

 the receptacle though probably formed at the same stage and in 

 the same way as in C. affinis, yet advances more rapidly, so that 

 in the young 1 1 mm. long it is comparable, in its closed up con- 

 dition, to C. affinis when 21 mm. long and in the sixth stage. 



Returning to C. affinis in the fifth stage, 15-18 mm. long, the 

 general appearance, Fig. 13, enlarged 13 diameters, shows an 

 increase in size of the animal and a great growth of the first 

 pleopods of the female which are now much longer than in the 

 female of the fourth stage and nearly as long as the male pleopods 

 of that stage, Fig. 10. 



The receptacle itself is quite diverse in different individuals. In 

 all there is added onto the median groove two elevations or 

 folds which tend to cover over the anterior end of the groove. 

 The groove itself is bent on one side more or less and the over- 

 hanging folds are more or less developed. In the specimen 

 shown in Fig. 14 as enlarged 200 diameters the groove bends to 

 one side and passes in under a marked flap or fold that grows 

 over the tip of the groove. This lateral fold may be called the 

 " hood " to distinguish it from the longer fold which passes along 

 the opposite side of the groove and tends to overhang it. 



