172 



PORIFERA 



CHAP. 



pointed at both ends it receives the special name oxea. The 

 lamination of the spicule is rendered much more distinct by heat- 

 ing or treatment with caustic potash. 1 



Fig. 66. Cut end of a length of a siliceous spicule from Hyalonema sieboldii, with the 

 lamellar structure revealed by solution. x 104. (After Sollas.) 



The archaeocytes are rounded amoeboid cells early set apart 

 in the larva ; they are practically undifferentiated blastomeres. 

 Some of them become reproductive elements, and thus afford a good 

 instance of " continuity of germ plasm," others probably perform 

 excretory functions. 2 



The reproductive elements are ova and spermatozoa, and are 

 to be found in all stages in the dermal jelly. Dendy states that 

 the eggs are fertilised in the inhalant canals, to which position 

 they migrate by amoeboid movements, and 

 there become suspended by a peduncle. 



The larva has unfortunately not been 

 described, but as the course of development 

 among the near relatives of H. panicea is 

 known to be fairly constant, it will be con- 

 venient to give a description of a " Hali- 

 chondrine type " of larva based on Maas' 

 account of the development of Gellius varius. 3 

 The free -swimming larvae escape by the 

 osculum ; they are minute oval bodies moving 

 rapidly by means of a covering of cilia. 

 The greater part of the body is a dazzling 

 white, while the hinder pole. is of a brown 

 violet colour. This coloured patch is non- 

 ciliate, the general covering of cilia ending at its edge in a 

 ring of cilia twice the length of the others. Forward move- 



1 Sollas, Ann. Mag. Nat. Hist. (4) xx. 1877, p. 285 ; Biitschli, Zeitschr. /. zviss. 

 Zool. xix. 1901, p. 236. 



2 Minchin, "Sponges" in Treatise on Zoology, edited by E. Ray Lankester, p. 

 87. See also Bidder, Proc. Roy. Soc. li. 1892, p. 474. 



3 Zool. Jahrb. Anat. vii. 1894. 



Fig. 67. Free-swimming 

 larva of Gellius vo.rius, 

 in optical section. , 

 Outer epithelium ; pi, 

 pigment ; x, hinder 

 pole. (After Maas. ) 





