No. 470] 



OLD AGE IN BRACHIOPODA 



115 



While thus the increase in the size of the animal becomes less 

 and less for each succeeding growth period, a time is reached, 

 varying with each individual, when another factor enters and 

 actual decrease or shrinkage begins. The tendency of the soft 

 parts of animals to contract in old age is familiar to us. (See 

 Hyatt, '96, p. 15; Quain, : 03, p. 1478). Through this tendency 

 can be explained many alterations in the hard parts which are 

 otherwise difficult of explanation. 



The soft parts and especially the mantle of brachiopods, as 

 well as of molluscs, are so closely related to the slu'll (Morse, :02, 



p. 321) that the least change in the former is expressed in the 

 latter. For example, a specimen of Laqucus calijornicus Koch, 

 No. 738, had the anterior portion of the mantle injured. The 

 scars occur in the same relative position on each valve, and the 

 mantle edge left a groove on the slieil, indicatint: tlic scar 'Fig. 

 26). Before the animal was injuml thr snrfaccf tiu> >lu'Il was 

 very smooth, showing: no sii;iis whatever of declinini^ >trcni:tli. 

 but as soon as the injury occunvd a lessened vitality is very notice- 

 able in the change in the angle of curvature and in the lamellose 

 growth lines, simulating senescence. 



A change in the angle of curvature of the shell shows that the 

 soft parts of the animal have ceased to grow a.s fast as formerly. 

 When, however, we consider such gerontic individuals as Athyris 



