depression and of the same number of cup-shaped depressions lying in 

 between them. However, this octoradial symmetry of the soft parts of 

 the disc is disrupted by the presence of the chitinous armature and the 

 entire system changes to a bilaterally symmetrical one (Fig. 274, B). 

 With this the middle hooks are located in the two posteriornnost lateral 

 septa, and in the latest changes in the structure of the disc in this 

 family they retain their location (as will be seen later in the other 

 families of the suborder). In connection with this, the evaluation of the 

 soft and chitinous parts of the attaching apparatus is considerably less 

 burdensome. As regards the edge hooks, apparently they are first 

 located near the margin of the disc directly in the region of the septa, 

 that is in places which have a more firm structure. This is probably 

 connected with the fact that the septa themselves are apparently historically 

 incepted at first in places of the distribution of the musculature and tendons 

 leading from the edge hooks to the center of the attaching disc which can 

 be determined on the basis of the structure of the ones developed in 

 Calceostomella and Dionchus. 



Further changes proceed along the direct line of the reduction 

 in part of the septa on the one hand and, conversely, in the appearance of new 

 secondary septa on the other, with the preservation of bilateral symmetry p. 365 

 and remnants of the initial octoradial plan of structure. Thus, the discs of 

 Dasybatotrema and Calicotyle , among which the lowest vertical septurr; 

 located between the septa where the middle hooks lie, is reduced are the 

 next evolutionary stage. Along with this, the location of the edge hooks also 

 undergoes a known compensating change which somewhat alters their 

 initial position and they are distributed more equally on the entire cir- 

 cumference of the disc, preserving at the same time the bulk of them toward 

 the posterior part of the disc, which is understandable from the functional 

 point of view because this part undergoes a considerably greater physical 

 load or stress than the upper one during attachment of the aninnal. With 

 this the edge hooks maintain their position in relation to the middle ones, i.e., 

 four of them remain along the lower edge of the disc between the 

 middle hooks, and the remaining ones lie along the upper edge of the disc 

 (Fig. Z74, C), Only part of the hooks remains along the edge of the septa, 

 whereas the remaining ones change into the interseptal space locating along the 

 marginal fringe of the disc. 



The appearance of a secondary ring of septa and of the corres- 

 ponding depressions along the marginal border of Ihe disc constitutes the 

 further change in the structure of the disc. With this, a depression forms 

 in front of each initial septum, i.e., the septum divides, so to speak, and 

 both of its halves separate. At the same time, between these two separated 

 septa one edges in, forming subsequently completely anew, and lies in front 

 of the recession between the two corresponding initial septa. As a result, 

 a system is formed from the central depression of the initial ring of 7 septa 

 with corresponding depressions, and from the secondary ring 18 septa with 



435 



