SPONGES 



8i 



or amphiblastula, in which liistogenesis has early taken place. The 

 larval period is very short, and fixation takes place by the anterior 

 pole, the riagellated layer becomes broken up and surrounded by 

 the inner mass. The pupal period, being occupied almost ex- 

 clusively by changes of a morphogenetic nature, is also greatly 

 abbreviated. The flagellated cells of the larva become arranged 

 to form the chambers ; the remainder of the sponge body arises 

 from the larval inner mass (Fig. 63, B and C). 





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Fio. 63. 



Tliree stages in tlie development of Axinella cristagalli, Maas. J, longitudinal section of the 

 larva ; B, early pupal stage soon after fixation ; C, late pupal stage shortly before the formation 

 of the osculum ; one half only of the section is represented, i.in, inner mass ; c.c, ciliated 

 layer ; d.l, dermal lay^'r ; '/.?, gastral layer ; fl.ch, flagellated chambers ; can.Ki/st, canal system. 

 (After Maas.) 



(y) Metaviorjyhosis and Organogeny. — Until the present decade it was 

 almost universally supposed that in all sponges except those with an 

 amphiblastula larva, such as Sycon, the ciliated layer of the larva 

 became the dermal epithelium (" ectoderm ") of the adult, while the inner 

 mas.s furnished the collared gastral epithelium (" endoderm ") and the 

 connective tissue layer (" mesoderm "). The only point at all disputed 

 was the origin of the flattened epithelium lining the gastral cavity and 

 the canals. Most authorities agreed with Schulze (1884) in deriving from 

 the " endoderm " the flat epithelium of the gastral cavity and of the 

 excurrent canals from the apopyles to the oscular margin, together with 

 the flagellated chambers themselves. The epithelium covering the exterior 

 and lining the incurrent canals up to the prosopyles was supposed, on the 



