ONTOGENY OF THE ANNULUS VENTRALIS. 131 



terior edge and thence to the anterior part of the groove. The 

 receptacle was now being formed of three different parts : the 

 original median groove, the two oblique folds. The former re- 

 mains as the posterior part of the adult sperm tube and the latter 

 help to make the orifice and the vestibule, or anterior part of the 

 adult receptacle. 



In all these cases the groove bends more or less to one side, 

 and in only one case observed was the bending to the observer's 

 right, which is the left of the animal. In this exceptional animal, 

 Fig. 17, not only does the groove bend to the right, but the hood 

 is on the right and the transverse fold and the posterior opposing 

 fold run at right angles to their usual course. Comparison of 

 Fig. 17 with Fig. 15, shows that they are, in the main, mirror- 

 images of one another. Each reversed, as seen through the 

 paper, would have the symmetry of the other. 



This reversal of symmetry in the receptacles of some young is 

 the first visible expression of the peculiar dimorphism of the an- 

 nulus in the adults of this and other species. While many of the 

 adults have the orifice upon the right side of the median plane 

 others have it upon the left and in all respects these two forms 

 of annulus are mirror-images of one another. Both forms are 

 used as sperm receptacles. Those with the orifice upon the right, 

 Fig. i, are more common ; in one lot of 41 females, 38 had 

 these right-handed annuli. 



The characteristic tuberosities of the adult grow out in later 

 larval life and they also are right- and left-handed in the following 

 way : while, usually, as in the above figure, the tuberosity upon 

 the left of the animal sends a ridge under the right tuberosity, 

 in left-handed adult annuli the behavior of the tuberosities is the 

 reverse. In the production of the two symmetrical adult forms 

 there is thus the harmonious development of the groove, folds 

 and tuberosities at different periods and from several areas of the 

 epidermis. 



In July the larva may pass into a sixth stage 2 1 mm. long. 

 On the ventral side, Fig. 18, enlarged thirteen diameters, there is 

 a noticeable increase in the longitudinal diameter of the annulus 

 and in the length of the first pleopods which are now longer than 

 those of the male of stage four, Fig. 10, though closely applied 



