DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 365 



condition, and figure 128 the spirals as developed in an indi- 

 vidual having a length of 2.5 mm., where the ribbon makes 

 but two revolutions. As far as can be ascertained, the loop 

 undergoes no essential modification in these early stages, 

 though its precise character in the example from which this 

 drawing has been made was not determined, but has been 

 drawn in. The same arrangement, however, has been seen 

 in an individual of but slightly larger growth. 



The growth of these spirals consists, primarily, in the addi- 

 tion to the number of coils, and, secondarily, in the thicken- 



126 



127 



128 



FIGURES 126-128. Development of internal apparatus in Homceospira evax 

 Hall. 



ing of the ribbon. In the first case the increase in number 

 must take place by addition to the apices of the coils, and 

 therefore the embryonic or primary coils of the ribbon must 

 be wholly concealed by later depositions upon them, both in 

 length and width. The apparent looseness of the coils in 

 their primary condition, however, must be regarded as largely 

 due to magnification, the distance between successive coils 

 being actually not so great as the distance between the apical 

 turns of the ribbon in the mature spiral. 



