1886.] POSITION AND CLASSIFICATION OF SPONQES. 565 



which the former consist of similar and equal, undifferentiated, cells, 

 which often remain isolated throughout life, whilst the latter pass 

 through a unicellular stage of short duration only, and consist, 

 when adult, of a numher of different cells. There is a vast dif- 

 ference between these two groups : the Protozoa are isocellular, 

 whilst the Metazoa are heterocellular. The Sponges are developed 

 in the same way as all other Metazoa and pass through the same 

 well-known embryonic stages — the Morula, Blastula, &c. They 

 consist, when adult, of a great number of differentiated cells. There 

 are flat epithelial cells all over the outer surface and on the canal- 

 wall ; there are collar-cells round the ciliated chambers. There 

 are gland-cells for different purposes, muscular and nervous cells 

 besides ordinary tissue and amoeboid cells in the Mesogloea or 

 ground-substance, in which also the ova and spermatozoa are 

 developed. It is therefore quite clear that the Sponges are not Pro- 

 tozoa, but Metazoa, and are, in fact, not similar to Protozoa in any 

 way. 



The Metazoa are naturally divided into two Groups or Grades — 

 the Coelentera, with a simple undivided body-cavity, all the parts 

 of which are in direct connection with one another ; and the Coelo- 

 mata, which have two distinct and entirely separated body-cavities — a 

 gastral cavity and a coelom or perigastric cavity. The Sponges 

 certainl)-^ have a simple and continuous body-cavity and no trace of 

 a coelom, so that they must be regarded as Coelentera. 



Long before Hertwig established the coelom theory, Leuckart 

 had already perceived this important fact, and placed the Sponges 

 among the Coelentera accordingly. 



Although nobody has ever attempted to regard the Sponges as 

 Coelomata, there has been great opposition, principally among English 

 authors, to Leuckart's opinion. I dismiss the arguments of those 

 who, like James Clark (284-294), Carter (166), and Saville Kent 

 (772), regard the Sponges as Protozoa, on the ground that their idea 

 of Protozoa does not harmonize with the generally adopted meaning of 

 the term, for if it did, they could not, as logical thinkers, count the 

 Sponges among them. Their idea of Protozoa comprises the whole 

 Animal Kingdom, because they draw no distinction between isocellular 

 and heterocellular organisms, and of course all Metazoa are, if this 

 distinction be omitted, colonies of unicellular Protozoa. F. E. 

 Schulze (1361) has taken the unnecessary trouble to refute Saville 

 Kent's (772) statements in detail, and to show that the latter had 

 been guilty not only of levity in the philosophical treatment o( his 

 work, but also of recording incorrect observations. 



Some very excellent men, particularly Balfour (17), Biitschli (138), 

 and Sollas (1440), are inclined to consider the Sponges as a separate 

 group equal in value to our groups Metazoa and Protozoa. This 

 arrangement was arrived at without regard to the division of the 

 Metazoa into Coelentera and Coelomata. They contrast the sub- 

 kingdom Porifera (Parazoa, Sollas) with the subkingdom Metazoa as 

 a whole. With all respect to the most important opiuion of Balfour, 

 I still do not see that there is any justification for the establishment of 



