Defense Mechanisms of Sponges, I — Cheng et al. 
401 
lishment of the types of cells in T. zeteki, it is 
now possible to examine the phagocytic role of 
each type. Furthermore, with the establishment 
of the normal ratios of each type of cell, we 
now have the baseline for determining whether 
increases of all or certain types of amoebocytes 
occur when challenged with foreign materials. 
These studies are currently in progress. 
SUMMARY 
The parenchymal cells of the sponge Terpios 
zeteki were studied both in histological sections 
and in smear preparations. Five distinct mor- 
phological types can be recognized. These are 
readily identified as collencytes, archaeocytes, 
thesocytes, chromocytes, and scleroblasts. The 
dimensions and morphological characteristics of 
each type are given. An endocytoplasmic sym- 
biotic zooxanthella occurs in the archaeocytes 
of T. zeteki. Counts revealed that collencytes 
are the most abundant of the five cell types. 
Many of these are syncytially arranged in situ. 
Archaeocytes are the next most abundant, 
followed by chromocytes, thesocytes, and sclero- 
blasts. Intracytoplasmic spicule formation was 
observed in scleroblasts. Having defined the 
cell types and ratios occurring in the mesoglea 
of T. zeteki, it is now possible to examine 
the role of each as associated with phagocytosis 
and other forms of internal cellular defense 
mechanisms. 
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