CLASSIFICATION. 133 



the anterior pole ; (2) rounded, more or less amoeboid elements, rarely 

 flagellated, forming the inner mass or aggregated at the posterior 

 pole; and (3) the archaeocytes, usually scattered in the inner mass, 

 and often represented by undifferentiated blastomeres. . . . 



"II. The larva fixes and undergoes a metamorphosis whereby the 

 flagellated cells become placed in the interior, while the cells of the 

 inner mass come to surround them completely. 



" III. (i) The flagellated cells of the larva become the choanocytes 

 of the adult (gastral layer), acquiring a collar ; . . . (2) the inner mass 

 gives rise to the dermal layer in its entirety.; ... (3) the archseocytes 

 become the wandering cells of the adult, from which the reproductive 

 cells arise." 



It is interesting to note that the primitive germ-cells are early set 

 apart. 



Classification. 



Class I. CALCAREA. With skeleton of calcareous spicules : 

 Grade I. Homoccela. Continuous internal layer of collared 



flagellate cells, e.g. Asceita, Leucosolenia. 

 Grade II. Heteroccela. Collared flagellate cells restricted to 



radial tubes or chambers, e.g. Sycon (Grantia). 



Class II. HEXACTINELLIDA, or Triaxonia, with sexradiate siliceous 

 spicules (triaxons). The members live chiefly in deep water, 

 e.g. Venus Flower-Basket (Euplectella) and the Glass-Rope 

 Sponge (Hyalonema). 



Class III. Demospongise. Skeleton of siliceous spicules, but never 

 triaxons, or of spongin fibres, or of spongin fibres and siliceous 

 spicules, or absent. 

 Grade I. Tetraxonida, typically with tetraxon spicules, e.g. 



Pachymatisma, Tetilla. 



Grade II. Monaxonida, with monaxon spicules, sometimes with 



spongin in addition, e.g. Mermaid's Gloves (Chalina oculata\ 



Crumb-of-Bread Sponge (Halichondria or Amorphina panicea), 



Fresh- Water Sponge (Spongilla). 



Grade III. Ceratosa, "horny" sponges with or without spicules, 



e.g. the Bath-Sponge (Euspongia). 

 Grade IV. Myxospongida, without any skeleton, e.g. Halisarca 



and Oscarella. 



A very remarkable form called Merlia seems to have both a siliceous 

 and a calcareous skeleton. 



History. Sponges, as one would expect, date back almost to the 

 beginning of the geological record. Thus the siliceous Protospongia 

 occurs in Cambrian rocks, and in the next series the Silurian the 

 main groups are already represented. From that time till now they 

 have continued to abound and vary. 



The division between calcareous and siliceous sponges goes deep 

 down to the very roots of the phylum, and the siliceous branch must 

 have divided very early into Triaxonia and Tetraxonia. 



(Ecology. Sponges are living thickets in which many 

 small animals play hide-and-seek. Many of the associa 



