vii SYSTEMATIC POSITION l8l 



two - layered animals ; with pores perforating the body -walls and 

 admitting a current of water, which is set up by the collared 

 cells of the " gastral " layer. 



Position in the Animal Kingdom. Sponges are the only 

 multicellular animals which possess choanocytes, and their mode 

 of feeding is unique. Since they are two -layered it has been 

 sought to associate them with the Metazoan phylum Coelen- 

 terata, but they are destitute of nematocysts or any other form 

 of stinging cell, and their generative cells arise from a class of 

 embryonic cells set apart from the first, while the generative 

 cells of Coelenterata are derived from the ectoderm, or in other 

 cases from the endoderm. These weighty differences between 

 sponges and that group of Metazoa to which they would, if of 

 Metazoan nature at all, be most likely to show resemblance, 

 suggest that we should seek a separate origin for sponges and 

 Metazoa. We naturally turn to the Choanoflagellate Infusorian 

 stock (see p. 121) as the source of Porifera, leaving the Ciliate 

 stock as the progenitors of Metazoa. 



That both Porifera and Metazoa are reproduced by ova and 

 spermatozoa is no objection to this view, seeing that the occur- 

 rence of similar reproductive cells has been demonstrated in 

 certain Protozoa (see pp. 100, 128). 



Let us now see which view is borne out by facts of embryo- 

 logy. Suppose, for the moment, we regard sponges as Metazoa, 

 then if the sponge larva be compared with the Metazoan larva 

 we must assign the large granular cells to the endoderm ; the 

 flagellated cells to the ectoderm ; and we are led to the anomalous 

 statement that the digestive cells in the adult are ectodermal, 

 the covering, outer cells endodermal ; or conversely, if we start 

 our comparisons with the adults, then it follows that the larval 

 ectoderm has the characters of an endoderm, and the larval 

 endoderm those of an ectoderm. 



Thus both embryology and morphology lead us to the same 

 point, they both show that in the absence of any fundamental 

 agreement between Porifera and Metazoa it is necessary to 

 regard the two stocks as independent from the very first, and 

 hence the name Parazoa (Sollas) has been given to the group 

 which contains the Porifera as its only known phylum. 



Interesting in connexion with the phylogeny of Parazoa is 

 the Choanoflagellate genus Proterospongia (Fig. 75), described by 



