THE CEYLON PEARL-OYSTER. 293 
(6) OBservaTions oN THE SrrucruRE oF THE FiBrous Cysts 
SURROUNDING THE CESTODE LARV& IN J/ARIGARTIFERA 
VULGARIS, 
T may here add a few notes on the structure of the investing 
eysts of Tylocephalum ludificans and 7’, ninus, based wpon my 
examination of Professor Herdman’s slides and Dr. Kelaart’s 
material, 
In the earliest stages the cyst may be scarcely differentiated 
from the surrounding tissue, and about 0:01 or 0-02 mm. thick. 
This condition I find specially associated with a little-developed 
phase of the smaller larva 7. minus, measuring 0°08 mm. in 
diameter, and resembling B in figure 4 on p. 21 of Part V. of 
Herdman’s Report. In such thin cysts the nuclei of the fibres 
are distinct, though hardly more so than those of the general 
connective tissue of the oyster. One or two examples at this 
and later stages were found in the muscular tissue without any 
surrounding cysts at all, beyond a little of the interstitial tissue 
of the muscle-bundles (Pl. XX XIII. fig. 2). As the fibrous 
capsule becomes thicker the nuclei appear ‘at first to become more 
abundant, and this may well be associated with the growth and 
multiplication of the fibres. As a rule, the thicker cysts (0°2 mm. 
thick and over) seem to be less densely nucleated, and may even 
show very few nuclei, especially when they become highly areolar 
and edematous. The outer part of the cyst is usually ordinary 
areolar connective tissue, with branched and anastomosing fibres 
passing over, often quite imperceptibly, into the general con- 
nective tissue of the body, such as occurs between the tubules 
of the liver. The cyst is, however, typically lined with several 
layers of more regular parallel fibres, with abundant nuclei. In 
some cases the fibres seem to coalesce to form a dense almost 
eristly substance, without obvious nuclei, 
The nuclei of the fibres are long and narrow, and are situated 
on their outside edges. 
In some cases the cyst appears to be entirely without nuclei, 
and in such cases the fibres are often very thick, measuring as 
much as 10 or 15 yu in diameter. 
There is never, so far as I can see, any trace of a lining 
epithelium, though the cyst may be lined with a layer of granules, 
possibly derived ‘from the Cestode or from the leucocy tes of the 
blood. 
In certain cases large dark-staining bodies were seen in the 
aveole of the fibrous connective tissue of the cyst, which suggested 
parasitic Sporozoa, but the preparations did not allow of detailed 
examination, 
A typical section of the cyst of the smaller parasite, 
Tylocephalum minus, is shown on Pl, XX XIII. fig. 3. 
This condition of things is very different to that which is found 
in the case of the pearl-inducing Trematode of J/ytilus described 
in my paper (25). Here the parasite is surrounded by an 
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