CONCRETIONARY GLAND. 
.S3.S 
Among the Streptoneuves tlie gland in the Diotocardia maintains 
its jirimitive position upon or near to tlie walls of the anrieles, the 
pericardial cavity being contracted by its proliferations, hut in the 
Monotocardia it is found upon the internal pericardial walls. 
The Concretionary Gland is characteristic of Cydostoma, although 
organs of somewhat similar aspect and probably analogous functions 
exist in Blthynia tentaculata and doubtless other species. The gland 
is to be regarded as a secretory organ and store-house for Uric acid 
and probably other waste substances, and was surmised by Garnault 
to have been acquired by the ancestors of the Cyclostomldcc when 
they relimpiished a(|uatic for terres- 
trial life, the more frequent periods 
of rest necessitated by the gveater 
ductuations of temperature, hindering 
the action of the kidneys, and the 
Uric acid accnmulating within the 
concretionary gland, to be afterwards 
re-absorbed and passed away from 
the system by the ureter when active 
life was resinned, as the concretionary 
gland has no excretory duct nor does 
it communicate either with the pallial 
'I'he gland is a greyish or brilliant 
white organ, occiqiying a dorsal position beneath the kidney and 
composed nf dense tufts of simple or branched follicles, some of 
which may he two millimetres in length, disposed among the intes- 
tinal convolutions, all being bmind together by loose connective 
tissue and encompassed by a rich vascular plexus. 
'J'his organ is variously developed, individually and seasonally, being 
more pronounced in autumn than in spring, the follicles at the former 
time being filled by multitudes of solid 
splimroidal concretions formed by concen- 
tric lamelhe, disposed around one or more 
nuclei. The colour of the gland is entirely 
dependent upon the number of concretions 
liresent in the follicles, the organ show- 
ing the characteristic brilliant white colour 
when they are numerous and being 
greyish when the contained concretions are less plentiful 
I’jG. — Follicle of Gland of Con- 
cretion of C^clostoma clegaiis (Mull.), 
highly magnified (after Garnault). 
cavity or the digestive tube. 
Fk;. G31. — Concretion from 
Concretionary Gland of Cyclo- 
sto/>in cU’gans (Mull.), highly 
magnified (after (xarnault), 
showing the concentric lamellar 
structure. 
