398 
Fig. 19. Syncytially arranged collencytes em- 
bedded in the mesoglea of Terpios zeteki. Drawn 
from section stained with hematoxylin and eosin. 
Notice unconnected collencytes intermingled with 
syncytially arranged ones. 
inclusions, which resemble stored nutrient 
globules, suggests their functional nature. There 
are two types of cytoplasmic inclusions. The 
more predominant is in the form of relatively 
large globose bodies each measuring approxi- 
mately 0.002 mm in diameter. The second type 
is in the form of small granules which are 
barely visible under the light microscope. The 
larger globules in living cells do not give a 
positive test for glycogen when stained with 
Lugol’s iodine but, when stained with Sudan 
Black B, only some give a positive test for 
lipids. In H and E stained sections, the cyto- 
plasm of thesocytes is eosinophilic. Some of the 
larger inclusion bodies are also eosinophilic, 
others appear yellowish, while still others are 
chromophobic. Among the smaller granules, 
some are eosinophilic while others are colorless. 
The nuclei, each measuring approximately 0.002 
mm in diameter, are strongly hematoxyphilic 
with distinct chromatin granules but no visible 
nucleoli. When observed in the living state, the 
larger inclusions vary from yellowish to color- 
less, while the smaller granules range from 
colorless to dark brown. 
The fourth type of parenchymal cell includes 
the chromocytes or pigment-bearing cells (Figs. 
16— 18). Actually two types of chromocytes, 
based on the color of the pigments present, are 
distinguishable: those enclosing red, and those 
with black, pigment granules. Both types of 
cells are of the same size, measuring 0.004 by 
0.003 mm (0.002-0.005 by 0.003-0.005 mm). 
These cells are distributed primarily in those 
regions of the mesoglea situated near the body 
surface of the sponge. Undoubtedly it is the red 
pigment-bearing chromocytes that give T. zeteki 
PACIFIC SCIENCE, Vol. XXII, July 1968 
its reddish surface color and the black pig- 
mented ones that give certain areas their black- 
ish color. Each cell includes a nucleus, measur- 
ing 0.0008 mm in diameter, with distinct 
chromatin granules but no visible nucleolus. 
The pigment granules are more or less evenly 
distributed through the cytoplasm. In H and E 
stained sections, these pigment granules are 
deeply hematoxyphilic. Living chromocytes are 
capable of amoeboid movement (Fig. 18). 
Skeleton-secreting cells or scleroblasts are 
rarely encountered in T. zeteki. Examination of 
sections of 10 sponges revealed only two, while 
in the numerous smear preparations examined, 
only nine scleroblasts, all silicoblasts (silicious 
spicule-forming cells), were observed. These 
cells, measuring 0.005 mm in greatest diameter, 
are quite similar to archaeocytes except for 
their slightly larger nuclei, which are devoid 
of nucleoli, and the presence of a developing 
spicule in each (Fig. 15). The young spicule 
is noncellular, colorless, and refractile. Its shape 
is generally elongate, with knobs at one or 
both terminals. In addition to the nucleus and 
young spicules, two types of cytoplasmic inclu- 
sions occur. The first is in the form of ovoid 
hematoxyphilic globules which are generally 
located near the nucleus. The second is repre- 
sented by extremely minute colorless granules 
randomly distributed in the cytoplasm (Fig. 
15). 
Cell Counts and Ratios 
A total of 23 differential counts were made 
of the cell types in T. zeteki by the method 
described earlier. The data are tabulated in 
Table 1. 
DISCUSSION AND CONCLUSIONS 
From our examination of both histological 
sections and dissociated cells it is evident that 
the parenchymal cells of Terpios zeteki can be 
identified as belonging to five cell types: col- 
lencytes, archaeocytes, thesocytes, chromocytes, 
and scleroblasts. These can be distinguished by 
their dimensions, cytoplasmic inclusions, and 
other characteristics. It is of interest to note that 
collencytes, which represent the most abundant 
type of cell, are capable of forming syncytia by 
the fusion of their fine pseudopodia. A similar 
