in PHYLUM AND CLASS PORIFERA 127 



(members of the order to which the Barnacles and Acorn-shells 

 belong) are invariably found embedded in certain species of Sponge. 

 Frequently a Sponge and a Zoophyte grow in intimate association, 

 so that they seem almost to form one structure. Thus the Glass- 

 rope Sponge (Hyalonema) is always found associated with aZoophyte 

 (Palythocu), and there are many other instances. Sponges often also 

 grow in very close association with certain low forms of plants 

 (Algae). 



The position of the Porifera in the animal series is unquestion- 

 ably among the Metazoa. The view that they are compound 

 Protozoa is now no longer maintained since the significance of 

 the facts of their development has been fully recognised. A 

 Sponge is to be regarded as a colony of Protozoa only in the sense 

 in which the same may be said of one of the higher animals. It 

 consists of a complex of cells, some of which have a consider- 

 able degree of independence, and some of which have a close 

 resemblance to certain Protozoa ; but the same is true of one of 

 the higher animals, the difference being one of degree and not 

 of kind. Like the rest of the Metazoa, the Sponge develops from 

 the oosperm by a process of yolk-division. 



But the Porifera are perhaps somewhat nearer the Protozoa 

 than are any of the other types of Metazoa ; and among the 

 Protozoa they appear to approach nearest to certain colonial 

 Flagellata. The genus Proterospongia (Fig. 58), already referred 

 to (p. 78), appears to be the member of the latter group which of 

 all known forms most closely resembles a sponge. Proterospongia 

 consists of a colony of collared Flagellates (Choanoflagellata) 

 embedded in a mass of gelatinous substance, in which there are 

 also amoeboid zooids similar to the amoeboid wandering cells of 

 Sponges. 



But, while the Porifera are clearly Metazoa, and not Protozoa, 

 there is some room for difference of opinion as regards their 

 relationships to the Coslenterata, with which great phylum they 

 have been sometimes amalgamated. The reasons for and against 

 such an arrangement will be discussed in considering the 

 general relationships of the Coalenterata. 



