COLD SPRING HARBOR MONOGRAPHS. VII. 



FOURTH FOOT 



Outer Ramus 



Outer Spines '. 3 



Apical Spine 1 



Apical Seta 1 



Inner Seta: 4 



Inner Ramus 



Outer Spine (Immature) 1 



Apical Spines 2 



Inner Setii; 3 



These terminal sections of the rami must break into two in 

 order to form the characteristic three-jointed rami of the adult. 



The older form had the following correlated characteristics: 

 Ten segments in the antennae and three segments in the rami of 

 the swimming feet. The distal segment of the older form is 

 armed as follows : 



Outer Ramus 



Outer Spines 3 



Apical Spine 1 



Apical Seta 1 



Inner Setae 3 



Inner Ramus 



Outer Spine 1 



Apical Spines 2 



Inner Set;£ 2 



This is the adult armature of the fourth swimming feet in C. 

 brevispinosus, and the experiment shows that the adult condition 

 is reached at an early stage in the life of the individual. 



Just what happens in the transformation from the two- jointed 

 to the three-jointed stage in the formation of spines I have not 

 been able to follow. 



This experiment threw no light on the relation of the individual 

 in the "parcus-stage" to C. brcz'ispijwsus, because the appendages 

 of the young were precisely like the parent forms as soon as they 

 reached the three-jointed condition. It showed, liowever, that the 

 characteristic spines of C. brevispinosus form at an early period, 

 and that if a spine is to be present, it is present as a spine from the 

 beginning. 



Again the young were reared from fertile females and the forms 

 were obtained figured in Plate V. The anterior feet are more 

 fully developed than the posterior ones, especially the fourth foot 

 which is, as yet, wholly unsegmented. 



Should these distal segments of the first and second rami sepa- 

 rate into two segments as was seen in C. parens, Fig. 2, Plate XL. 

 it would result in the formation of a "parcus-stage" having two 

 outer spines on the distal segments, in place of three. I have, 

 however no proof that such a separation occurs without the forma- 

 tion of additional spines. 



