234 



PORIFERA 



originate as spheres (Fig. 119, B) ; and spheres have been observed 

 in the gemmule of a Tetliya ; no spherasters vs^ere as yet present in 

 the gemmule, and spheres were absent in the adult.^ 



In the genus Placospongia certain spicules are present which 



outwardly closely resemble the 



sterrasters so characteristic of 

 certain Tetractinellidae. Their 

 development, however, as will 

 be seen from Fig. 120, shows 

 that they are not polyaxon but 

 spiny monaxon spicules. Placo- 

 spongia is consequently trans- 

 ferred to the Monaxonida Spin- 

 tharaphora. 



Sterrasters originate within 

 an oval cell as a number of 



Fig, 121. — Three stages in the development , . ,., „, •>,,■■,■, % , ■ , 

 of an anisochela. al, Ala; at, lower hair like fibres "^ (trichltes), whlch 



aia;/, faix;/', lower faix; r, rostrum ; are United at their inner ends. 



r , lower rostrum. (After Vosmaer and i . i t 



Pekeiharing.) The outer ends become thickened 



and further modified. The 

 position occupied by the nucleus of the scleroblast is marked in 

 the adult spicule by a hilum. 



The anisochela has been shown repeatedly to originate from a 

 C-shaped spicule.' 



What little is known of the development of Hexactinellid 

 spicules we owe to Ijima.* Numerous cells are concerned in 

 certain later developmental stages of the hexaster; a hexaster 

 passes through a hexactin stage, and — a fact " possibly of 

 importance for the phylogeny of spicules in Hexactinellida " — 

 in two species the first formed spicules are a kind of hexactin, 

 known as a " stauractin," and possessing only four rays all in one 

 plane (c£ Protospongia, p. 207). 



Physiology 



Production of the Current. — It is not at first sight obvious 

 that the lashing of flagella in chambers arranged as above 



^ I. Sollas, P. Zool. Soc. London, ii. 1902, p. 215. 

 2 Sollas, Ann. Mag. Nat. Eist. (5) ix. 1880, p. 402. 



^ Bowerbank, and also Vosmaer and Pekeiharing, Verh. Ah. Amsterdam (2) vi. 

 3, 1898. 



" J. Coll. Japan, xv. 1901, p. 193. 



