234 



PORIFERA 



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

 in the gemmule of a Tethya ; no spherasters were 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, howe^'er, 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 

 hairlike fibres^ (trichites), which 

 are united at their inner ends. 

 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 (cf. Protosponr/in, p. 207). 



Fig. 121. — Three stages in the development 

 of an anisochela. al, Ala ; al', lower 

 ala ;/, falx ; ,/'', lower falx ; r, rostrum ; 

 r', lower rostrum. (After Vosmaer anil 

 Pekelharing.) 



Physiology 



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

 that the lashing of flagella in chambers arranged as above 



1 I. Sollas, P. Zool. Sue. London, ii. 1902, p. 2ir.. 



•- Sollas, Aim. Mag. Nat. Hist. (5) ix. 1880, p. -102. 



^ Bowerbank, and also Vosmaer and Pekelharing, Vcrh. Ale. Amsterdam (2) vi. 



3, li 



* J. Coll. Jaimn, xv. 1901, p. 19.3. 



