THE CEYLON PEARL-OYSTER. PEC 
plates, the concentric stratification of the majority of which 
never occurs in a Trematode nucleus, and could hardly be expected 
in a Cestode. 
Text-fig. 33. 

A Mytilus pearl, from Foulney, near Piel, Lancashire, with a Trematode nucleus : 
A, examined entire in oil of cloves, after decalcification ; B, the same in section. 
In A the oral sucker and digestive ceca of the worm are distinctly visible. 
In B the internal anatomy is still preserved. cw., cuticle; sp., spines on 
same; dig., digestive cecum ; skr., ventral sucker; at dand g ave dark masses, 
which may well represent the remains of the yolk-glands and gonads; 
nac., nacre. C,a Mytilus pearl of non-Trematode origi, from Plymouth. 
Here the nucleus is, as in many Ceylon pearls, a minute group of granules. 
A X20; B xX 70; € xX 20. 
As Mr. Cyril Crossland, Marine Biologist to the Sudan Govern- 
ment, is quoted by Professor Herdman (Report Ceylon Pearl 
Fisheries, Pt. V. Pearl- Production, p. 3) as supporting the Cestode 
theory, so far as M/. vulgaris in the Red Sea is concerned, I wrote 
to him to ask him for further information. He replied, in 
a letter dated December 9th, 1911 :—‘‘I never published any state- 
ment that Cestode larvee caused pearl-formation in the Red Sea. 
‘Lhe evidence to my mind is in need of revision. In all cases the 
first result of excessive stimulation of the secretory epidermis of 
the mantle is the formation of a dark brown horny material 
[7.e. my amorphous substance.—H. L. J.].. How would this stain 
in sections, and is it cellular like the horny material of the prismatic 
layer? If so, would not a shrunken nucleus of such material 
resemble the dry remains of a Cestode 2? This is a criticism which 
I have had in mind several years, and have never put it to 
the test.” From this it is clear that Mr. Crossland, though cut 
off from the possibility of applying modern laboratory technique, 
has arrived at much the same conclusion as that which I am 
elaborating in this paper. 
To summarise the supposed relation between Cestodes and 
[19] 
