Development of the Calcispongias. 4o 



posed of two dissimilar halves (only one of which is clothed 

 with cilia). But as, according to Hackel, the course of deve- 

 lopment agrees in essentials in different Calcispongia?, we must 

 deal with his representation as general. 



With regard to the first stages (embryonic development), 

 my observations are in accordance with those of Hackel ; but 

 this only renders the difference as to the later states more 

 remarkable. Besides that according to Hackel the internal 

 cavity in the larvae is always lined with a particular layer of 

 spherical cells, whilst Oscar Schmidt and myself detected 

 nothing of the kind, Hackel's description deviates most 

 widely from mine with respect to the postembryonic deve- 

 lopment. He makes no mention either of the invagination of 

 the ciliated layer or of the hypertrophy of the unciliated 

 layer ; the whole metamorphosis is supposed to be reduced to 

 this — that the ciliated exoderm retracts its flagellar filaments 

 and becomes converted into the so-called " syncytium," while 

 the entoderm of the larva acquires cilia in order to furnish 

 the so-called flagellate epithelium. The reason why these 

 views are so directly opposed to mine is easy to find, if we 

 carefully peruse the chapter on the developmental history of 

 the Calcispongia? (pp. 328-338). From this it appears that 

 Hackel never observed the postembryonic development in the 

 sponges, but has invented it a priori. The following passage 

 is very instructive : — " The conversion of the swimming 

 Gastrula into the youngest and simplest attached state, which 

 we will call Ascula, appears to take place very rapidly and 

 has not yet been observed. The changes occurring therein 

 may ^ however, be directly inferred from the comparison of the 

 Ascula an d Gastrula (!). The attachment of the latter takes 

 place at the aboral pole of the longitudinal axis, at the end 

 opposite to the buccal orifice. The flagellate cells of the 

 dermal lamella suspend their vibratile movements, retract 

 the flagellar process, and lose their slender cylindrical form, 

 flattening and spreading out into the extending intestinal 

 surface. The unciliated entodermal cells, on the other hand, 

 divide repeatedly and then become converted into flagellate 

 cells, each of them extending a filiform flagellum from its 

 proximal end, or that turned towards the stomachal cavity " 

 (p. 337). But even this is not all ! Hackel says that he 

 has " inferred " (erschlossen) the metamorphosis of the Calci- 

 spongise from the comparison of the Gastrula with the Ascula 

 (i. e. " the youngest and simplest attached stage ") ; but no- 

 where does he give either a description or a figure of even a 

 single actually observed Ascula ! From this we may infer 

 that he has really never seen an Ascula ; for otherwise he 



