THE BRACHIOPODA 11 



which have a punctate shell, each punctation contains a 

 minute outgrowth of the mantle, and in at least one 

 recent species this has been shown to have the structure 

 of a simple sense-organ, though the nature of the sense 

 is unknown. m 



From the egg the larval brachiopod escapes as a 

 microscopic body covered with vibratile cilia, by means 

 of which and the action of currents it gets the only 

 chance of its life to migrate away from the fixed home of 

 its parents. It is not likely that any one generation 

 travels very far, yet some modern species are world- wide, 

 thanks to the cumulative effect of small journeys through 

 thousands of generations. The length of life of brachio- 

 pods in general is not known, but in one species there is 

 evidence of five years' life. 



Very soon the larva fixes itself by a rudimentary 

 pedicle and begins to secrete two valves. In all observed 

 cases this first shell, or protegulum, has the same general 

 form, which is also the form of the most primitive 

 brachiopod shells in the Lower Cambrian rocks (Fig. 6, a). 

 This resemblance favours the view that the development 

 of the individual (ontogeny) repeats, in an abbreviated 

 and more or less imperfect manner, the ancestral history 

 of the race (phytogeny). This principle of recapitulation, or 

 palingenesis, is of the utmost importance in palaeontology 

 as a criterion of blood-relationship, and especially in the 

 case of those animals whose shell-growth is of the kind 

 found in brachiopods, where the early stages of the shell 

 are preserved and added to, to form the later (growth by 

 accretion). 



