240 



Olynthus change their character or are replaced partially by choa- 

 nocytes, partially by pinacocytes. There is nothing extraordinary 

 in this supposition, since Maas') and myself") have stated how 

 the flagellated cells of larvae are transformed into pinacocytes in 

 the young fixed sponge , and that various transitions between pi- 

 nacocytes and choanocytes exist all along the lines of demarca- 

 tion between these two. 



It is now only a logical conclusion to say that the main ex- 

 current tubes of L. stipitata, L. tripodifera and a Sycon (say 

 e. g. S. ciliatum) are homologous formations. We may add even 

 the Hexactinellidae. All these are the simplest modifications of 

 the original Olynthus-sack without diverticula. 



This central cavity of the sponges , just named , / call cloaca , 

 and its aperture , osculum {s. s.). 



In the wall of the cloaca there is a tendency , if not very 

 markedly developed , to form a special skeleton, In other terms, 

 we find very frequently indeed , a cloacal skeleton as well as a 

 dermal one. Now it will be clear , that the more this cloacal ske- 

 leton developes, the more we shall find definite openings in it, 

 where the diverticles debouch. The openings of canals or lacunae 

 into the cloaca I call procts. I will at once agree that thus in 

 many Syconidae the procts coincide with the apopyles. But these 

 are of course exceptional cases , comparable to the coalescence of 

 pores and prosopyles in the simplest Ascon. 



In Hexactinellids we see that the diameter of the cloaca varies 

 a good deal. And so does its length. In fact we see a series of 

 transitions from a pure cylindrical form to a funnel- or cup-sha- 

 ped form, 1 only wish to draw attention to sponges as on one 

 side Holascus, Euplectella, Malacosaccus, thin- walled cylindrical 

 tubes, and Crateromorpha on the other side. We find transitions 

 to both extremes in Sympagella , Polyrhabdus , Balanites , Phero- 



1) Maas O. Zur Mctaraorphose der Spongillalarve. In: — Zool. Anz. Jahrg. 12. 

 1889 (Prei. account. The paper in Z. W. Z. Bd. L. 1890.) 



2) VosMAKK G C. .1. Note on the metamorphosis of the sponge-larva. In: — ïijdschr. 

 Nederl. Dierk. Ver. (2) Deel 2. 1889. 



