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PACIFIC SCIENCE, Vol. XXII, July 1968 
parenchymal cells were made as these revealed 
certain distinguishing characteristics not readily 
observable in stained sections. For example, the 
pseudopodial movements of each type of cell 
and their cytoplasmic inclusions, especially the 
pigments of chromocytes, were clearly visible 
only in living cells. The dimensions of the cells 
were determined from living cells by use of a 
calibrated ocular micrometer. 
The percentages of the total number of 
parenchymal cells represented by each cell type 
were determined in the following manner. Uni- 
form suspensions of dissociated cells of 10 
sponges were made in filtered sea water. Sam- 
ples of such suspensions were examined with a 
phase-contrast microscope equipped with a 
Whipple-Hausser ocular micrometer to facili- 
tate counting. During the counting procedure, 
the first hundred cells recognized as amoebo- 
cytes native to the mesoglea encountered in 
the squares were recorded by type. A total of 
23 counts were made. 
RESULTS 
Descriptions of Cell Types 
The architecture of Terpios zeteki is of the 
leuconoid type. The viscous mesogleal layer fills 
the spaces between the flagellated chambers and 
canals and is tightly packed with cells and 
spicules. The most abundant type of cell is the 
small spherical collencyte, each measuring 0.003 
mm (0.002-0.004 mm) in diameter (Figs. 1- 
5). When examined in histological sections, 
many of the collencytes possess fine pseudo- 
podia which are fused with those of adjacent 
cells to form a netlike syncytium (Fig. 19) with 
independent cells intermingled with it. In prep- 
arations of dissociated cells, however, collen- 
cytes generally round up, appearing as minute 
spheres which are not interconnected. Occa- 
sionally one is seen producing one or more fine 
pseudopodia (Fig. 4). In hematoxylin-and- 
eosin stained sections, the cytoplasm of collen- 
cytes is agranular and is either chromophobic or 
only slightly eosinophilic while the nucleus is 
homogeneously slightly hematoxyphilic. These 
cells are limited to the parenchyma although 
occasionally collencytes have been observed free 
in the water canals, particularly excurrent canals. 
When dissociated living collencytes are observed 
under the light microscope, the cytoplasm ap- 
pears clear but the nucleus is extremely difficult 
to define. When examined with phase-contrast 
microscopy, however, each nucleus appears to 
be homogeneous and rounded. On a few occa- 
sions the nuclei of collencytes have been 
observed dividing although the exact mitotic 
figures have not been studied (Fig. 5). 
The second most abundant type of paren- 
chymal cell is the rounded to ovoid archaeocyte 
which measures 0.006 by 0.005 mm (0.004- 
0.007 by 0.004-0.007 mm) (Figs. 6-11). 
This type of cell appears either rounded or with 
a lobose pseudopodial projection in histological 
sections. Many of them enclose a rather large 
globose to ovoid hematoxyphilic body that 
measures 0.003 mm in greatest diameter and 
which when observed in the living state proved 
to be a yellowish-green symbiotic zooxanthella 
with a distinct bright red stigma. In addition 
to the zooxanthella, each archaeocyte includes 
a rounded nucleus with a distinct nucleolus, 
and some include cytoplasmic inclusions which 
vary in size, while others include one or two 
vacuoles. The cytoplasm of these cells is faintly 
eosinophilic while the nucleus is hematoxy- 
philic. It should be noted that very rarely the 
pseudopods of two adjacent archaeocytes may 
be fused to form a two-celled syncytium. In 
addition, archaeocytes are occasionally found 
free in the water canals, primarily the excurrent 
canals. When examined in the living state, 
archaeocytes are either spherical or ovoid. Ap- 
proximately 45-50% of those encountered in- 
clude endosymbiotic zooxanthellae. The cyto- 
plasm may be clear or with a few hyaline 
inclusions. These cells have been observed to 
produce a single lobopodium (Figs. 9, 10). 
Occasionally some have been observed under- 
going what appears to be division (Fig. 11); 
this, however, is not a common phenomenon. 
Approximately one out of 60 cells is dividing. 
The third type of parenchymal cell encoun- 
tered in X. zeteki is believed to represent theso- 
cytes. Each of these measures 0.007 by 0.006 mm 
(0.005-0.010 by 0.004-0.009 mm). According 
to Hyman (1940), thesocytes are nutrient- 
enclosing cells. The exact chemical nature of 
the cytoplasmic inclusions of what we are desig- 
nating as thesocytes has not been determined 
although the consistent occurrence of these 
