AFFINITIES OF THE SPONGIDA. 33 



ordinary Rhizopoda most nearly allied to the Amoebina, which exhibit a 

 like modification of structure with relation to the latter as is presented 

 by the Opalinidde with respect to the ordinary Ciliata. The much-vexed 

 question of the zoological position and affinities of the Spongida or Porifera 

 has necessarily to be considered in association with the delimitation of the 

 sub-kingdom Protozoa. Formerly the members of this important section 

 were regarded mostly as forming either a subordinate group of the Rhizo- 

 poda, or an independent class of the Protozoa. More recently, however, 

 there has been a tendency to exclude the sponges entirely from the Protozoic 

 sub-kingdom, and to assign to them a position more nearly approximating 

 that of the Coelenterata, or zoophytes and corals, among the more highly 

 organized tissue-constructed animals or Metazoa. Professor Ernst Haeckel, 

 the most powerful supporter and also the originator of this proposed 

 innovation, has based his arguments in favour of such transfer chiefly upon 

 his own peculiar interpretation of the structure and developmental phe- 

 nomena of those bodies, the swarm-gemmules or so-called ciliated larvae, 

 hereafter described, by which the local distribution of special sponge 

 species is periodically effected. Taking on trust this developmental inter- 

 pretation of Ernst Haeckel, many leading biologists have committed 

 themselves to a similar exclusion of the Spongida from the Protozoa, 

 and it is thus that in Professor Huxley's recently quoted work — which 

 must be accepted as the latest and most important exposition of Inverte- 

 brate anatomy in this country — a like allocation of this much-debated 

 group to the Metazoic section of the animal kingdom is upheld. Postponing 

 for a future chapter a complete summary of the grounds upon which an 

 interpretation entirely opposed to that advocated by Professor Haeckel is 

 adopted in this volume, it will suffice for present purposes to state that a 

 considerable interval devoted to a careful investigation of the structural and 

 developmental phenomena of the sponges and Protozoa generally has 

 resulted in the arrival by the present author at the opinion that — (i) these 

 phenomena accord essentially and entirely with those presented by the 

 typical Protozoa ; (2) that there is no formation of a germinal layer or true 

 tissue structure in any period of their development ; and (3) that the posi- 

 tion of the Spongida among the Protozoa is most nearly allied to that 

 Infusorial group here distinguished by the title of the Choano-Flagellata, 

 and out of which, by the process of evolution, there is substantial reason 

 to presume they were primarily derived. 



Proceeding with the consideration of the subdivision of the Protozoa into 

 subordinate classes and orders, it has been further found, in association with 

 the investigations above referred to, that the older and primary groups of 

 the Rhizopoda and Infusoria, or of the Myxopoda and Mastigopoda, as 

 more recently proposed, by no means allow of as clear and natural a 

 grouping of their various orders as it is possible to submit, while it is still 

 less efficacious for the indication of the many complex affinities that 

 undoubtedly subsist between one and another, or, as it is often found, 



D 



