METAZOA PORIFERA. 75 



genera with skeletal structures which has reached its 

 present condition by reduction, or whether it is an existing 

 representative of a primitive type which has never had any 

 skeletal structures. Whatever way the result has been 

 arrived at, the existing Halisarca is obviously a skeleton- 

 less kind of silicious sponge, and can be used to show what 

 such forms are like. It is' a fleshy animal with the typical 

 characters of pores, canals, ampullaceous sacs, and cloa- 

 cal opening. The genus is interesting because it increases 

 not only by eggs but also by a process known as budding 

 which is essentially the same as that of division so com- 

 mon among the Protozoa. In this case, however, there 

 is not an equal division of the body, but a part separates 

 from the rest and becomes a new animal. 



SILICEA. HEXACTINELLIDA. 



This group is represented by fossils, and living mem- 

 bers are mostly found in the deep seas. Although the 

 oldest silicious sponges probably possessed separate spic- 

 ules, yet on the death of the animal these would fall apart 

 and be swept away and deposited along with other re- 

 mains, so that no satisfactory inferences can be drawn in 

 regard to the sponges possessing them. 



The predominating six rayed spicules of the group 

 have been shown to be simply a modification of the three 

 rayed form which we have found among the Calcarea. 

 It has also been proved that the anatomical structure and 

 the development of these sponges are in some ways like 

 those of the calcareous sponges. For these reasons, and 

 because the group is found in the oldest geological forma- 

 tions, the Hexactinellida are considered as the more gen- 

 eralized of the silicious sponges. 



Ventriculites (No. 68), often occurring in the chalk, is 

 made up of six rayed spicules which are always fused 

 together. It is more or less cup-shaped with a wide cen- 



