46 



INVEETEBEATA 



CHAP. 



dermal cells, and these masses become hollowed out to form the 

 spherical flagellated chambers. 



A B 



mes 



mes 



FIG. 23. Two sections of the body-wall of the larva of Plakina monolopha in order to 



show the distinction between archaeocytes and mesenchyme. (After Maas.) 

 A, a piece of wall of embyro not yet hatched. B, a piece of wall of free larva ; arch, archaeocytes ; 



mes, mesenchyme. 



In this series, Grantia forms, not the beginning, but takes the 

 second place, and, viewing the series as a whole, we see a 



progressive shortening of the 

 larval life joined to an 

 anticipation of adult char- 

 acters. We have, indeed, 

 before us, typical examples 

 of the commonest form of 

 the modification of develop- 

 mental history from its 

 primitive form. This consists 

 in the reflecting back of 

 structures characteristic of 

 one period of the life-cycle 

 to successive earlier periods 

 in ontogeny. It is called 

 heterochrony, and its pos- 

 sible cause will be discussed 

 in the summary. The merit 

 of having called attention to 

 it, and of having emphasised 

 its importance, belongs to 

 Lankester. 



The development of the 

 most primitive sponges, the 

 Asconidae, has been worked 

 out by Minchin (1896), and 

 his results are of great 

 interest but a little difficult to reconcile with the series determined 

 by Maas. In the genus Clathrina, the embryo is hatched as an 



FIG. 24. Longitudinal section through the Amphi- 

 blastula larva of Esperia lorenzi. (After Maas. ) 



fl, flagellated cells; gr, granular cells; mes, "inesen- 

 chyme";'scZ, scleroblasts secreting curved spicules ; 

 spi, shovel-shaped spiculea ; sp'*, pin-head spicules. 



