THE P11YBIOLOUY OF ASCIDIANS
41
This rytoniiclear cycle is most pronoiiiiced in that portion of the
gland adjacent to the mid-gut! itnd k‘oriqiie ( 1 954) describes how the
epithelium of the intestine is modified in this area so that the cells
become flatter und more basopliile than elsewhere in the intestine and
have characteristics similar to those of the intact cells of the pyloric
iimpullw. In addition Fouque describes free cells in this area which he
has termed “ parapyloric cells ” ; these cells are believed to have their
origin usually from the cells of the ampulla, but occasionally, e.g. in
Botryllus, they may arise froin t h e intestinal epithelium, while in
Diplosoma they are absent but the pyloric vesicle is stated to be surrounded by “ haemolymph ” cells. The role of the parapyloric cells is
further discussed below.
In the more distal parts of the gland the cytonuclear cycle is less
pronounced and the ampullae frequeritly contain concretions which
Fouque believed to be formed through the heukdown of haemolymph
cells which have migrated into the ampulla. The concretions are often
bi-refringent and may be partly purine in nature. Az6ma believed that
in Aacidiidae the cytonuclear cycle did not occur and he described
hyaline concretions in the ampullae. Concretions have been described
in the pyloric gland by several authors including Chandelon (1875),
lsert (1903), Millar (1949, 1953a) but are not mentioned by Colton
(1910). Isert believed they arose from masses of matter formed by the
iwcumulation of secreted substances, while M i l k considered that they
inay crystallize out from the liquid contents of the tube.
The physiological role of the pyloric gland is &ill a matter of speculation but most authors have ascribed to it either an excretory or a
digestive function or both, but throughout the literature there is
insufficient evidence of adequate experimental work to enable any
definitive statements to be madr. The older literature is reviewed by
A d m a (1937), Fouque (1954) iind Millar (1963a) and the reader is
referred to them for further detail.
The evidence for an excretory role is based partly on the preeence of
solid concretions in the ampullae ttnd comparisons have been made be.
tween the storage excretion of the nephrocytes and renal vesicles of many
species and the possible purine nature of the concretions. However, the
deposition of purine granules appears to be a widespread phenomenon
ill wcidians and the quantities deposited (if any) in the pyloric gland
would be too small to be of any overall significance ih the animal’s
metabolism. If purines are deposited in the gland this must be a
secondary function and possibly even accidental.
The pyloric gland will also pick up vital dyes (Fouque, 1954;
Godeaux, 1954) and thorium dioxide (Brown and Davies, 1971) from
41
This rytoniiclear cycle is most pronoiiiiced in that portion of the
gland adjacent to the mid-gut! itnd k‘oriqiie ( 1 954) describes how the
epithelium of the intestine is modified in this area so that the cells
become flatter und more basopliile than elsewhere in the intestine and
have characteristics similar to those of the intact cells of the pyloric
iimpullw. In addition Fouque describes free cells in this area which he
has termed “ parapyloric cells ” ; these cells are believed to have their
origin usually from the cells of the ampulla, but occasionally, e.g. in
Botryllus, they may arise froin t h e intestinal epithelium, while in
Diplosoma they are absent but the pyloric vesicle is stated to be surrounded by “ haemolymph ” cells. The role of the parapyloric cells is
further discussed below.
In the more distal parts of the gland the cytonuclear cycle is less
pronounced and the ampullae frequeritly contain concretions which
Fouque believed to be formed through the heukdown of haemolymph
cells which have migrated into the ampulla. The concretions are often
bi-refringent and may be partly purine in nature. Az6ma believed that
in Aacidiidae the cytonuclear cycle did not occur and he described
hyaline concretions in the ampullae. Concretions have been described
in the pyloric gland by several authors including Chandelon (1875),
lsert (1903), Millar (1949, 1953a) but are not mentioned by Colton
(1910). Isert believed they arose from masses of matter formed by the
iwcumulation of secreted substances, while M i l k considered that they
inay crystallize out from the liquid contents of the tube.
The physiological role of the pyloric gland is &ill a matter of speculation but most authors have ascribed to it either an excretory or a
digestive function or both, but throughout the literature there is
insufficient evidence of adequate experimental work to enable any
definitive statements to be madr. The older literature is reviewed by
A d m a (1937), Fouque (1954) iind Millar (1963a) and the reader is
referred to them for further detail.
The evidence for an excretory role is based partly on the preeence of
solid concretions in the ampullae ttnd comparisons have been made be.
tween the storage excretion of the nephrocytes and renal vesicles of many
species and the possible purine nature of the concretions. However, the
deposition of purine granules appears to be a widespread phenomenon
ill wcidians and the quantities deposited (if any) in the pyloric gland
would be too small to be of any overall significance ih the animal’s
metabolism. If purines are deposited in the gland this must be a
secondary function and possibly even accidental.
The pyloric gland will also pick up vital dyes (Fouque, 1954;
Godeaux, 1954) and thorium dioxide (Brown and Davies, 1971) from