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1886. ] POSITION AND CLASSIFICATION OF SPONGES, 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 number 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 ameeboid cells in the Mesoglcea 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 Metuzoa, and are, in fact, not similar to Protozoa in any 
way. 
The Metazoa are naturally divided into two Groups or Grades— 
the Ceelentera, with a simple undivided body-cavity, all the parts 
of which are in direct connection with one another; and the Ceelo- 
mata, which have two distinct and entirely separated body-cavities—a 
gastral cavity and a ccelom or perigastric cavity. The Sponges 
certainly have a simple and continuous body-cavity and no trace of 
a ccelom, so that they must be regarded as Ccelentera. 
Long before Hertwig established the ccelom theory, Leuckart 
had already perceived this important fact, and placed the Sponges 
among the Ceelentera accordingly. 
Although nobody has ever attempted to regard the Sponges as 
Ccelomata, 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 of 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 Ccelentera and Ccelomata. They contrast the sub- 
kingdom Porifera (Parazoa, Sollas) with the subkingdom Metazoa as 
a whole. With all respect to the most important opinion of Balfour, 
I still do not see that there is any justification for the establishment of 
