THE PfIYSI01,OGY OF ASOIDIANS
123
appews to havt? t i direct relationship to the complexity of the food
collecting appiiratus. In the aplounobraiichs the ciliated opening is
usually a simplo oval apcrburtt ; in phlcbobranchs there is a tendency
towards foldiiig to form a C-shtiped aperture; in the more complex
stolidobranchs, particularly in Pyuridae, the horns of the C tend to be
rolled inward into a spiral structure or to form other complex patterns.
The overall effect of sucb increaaing complexity is to increase the length
of the ciliated border of the funnel. The organization and position of
the ciliated funnel are thus such that it could be used to “inject ”
cellulur materid into t h e beginning of the food cord and the question
should be invedigated further.
XIII. THE DORSAL STRAND
I n the adult uscidian the duct of the neural gland extends posteriorly
(t8 a fine strand of tissue, the dorsal strand or dorsal cord. A short
distance behind the gland it leaves the mantle and crosses to the dorsal
wall of the pharynx where it runs close above the dorsal lamina.
Posterior to thcb branchial sac in enterogonous aacidians (i.e. Ciona) it
extends to the gonad where it penetrates into the ovary and terminates
abruptly as a slight swelling (Millar, 1963s; Aubert, 1954). In pleurogonous ascidiaiis in which the gonads are in the mantle wall (i.e.
Molgula) the strand terminates abruptly in the neighbourhood of the
digestive diverticultl of the stomach (Van Beneden and Julin, 1884).
It is gonerally tigreed that this strand is derived from that part of the
larval neural tube which also gives rise to the neural gland and ganglion
(Willey, 1893 ; Van Beneden and Julin, 1884 ; HUUS, 1924 ; Brien, 1927 ;
Berrill. 1960; Millar, 1953a; Aubert, 1964).
Although the strand is derived from embryonic neural tissue there
are no nerve cells or fibres in it. In Molgula manhatten& Van Beneden
and Julin (1884) described the strand as composed of large cells resembling the large cells of the cerebral ganglion and arrangedradially around
a central core or small cavity. However, these authors pointed out that
the cells in M. rnrsnhattensia were larger than in many other species
examined, and Millar (l963a) describes the cells in Ciona intestinalis as
“ quite small, and spindle shaped, being somewhat elongated along the
strand ”. In Ciona the whole strand ie only 10 p to 16 p in diameter.
Huus (1924) does not figure the cord in section but his illut3trations
show rounded cells scattered unevenly in the strand of Corella paralldogramma (Muller). Brien (1927) describes them in the following term8
for Aplidium, pallidurn : ‘‘ Lea cellulen sont itlentiques 8, celles du canal
t’xcrdteiir, ou plug extlctement aux amoeboc*yten clu sang avec lesquek
123
appews to havt? t i direct relationship to the complexity of the food
collecting appiiratus. In the aplounobraiichs the ciliated opening is
usually a simplo oval apcrburtt ; in phlcbobranchs there is a tendency
towards foldiiig to form a C-shtiped aperture; in the more complex
stolidobranchs, particularly in Pyuridae, the horns of the C tend to be
rolled inward into a spiral structure or to form other complex patterns.
The overall effect of sucb increaaing complexity is to increase the length
of the ciliated border of the funnel. The organization and position of
the ciliated funnel are thus such that it could be used to “inject ”
cellulur materid into t h e beginning of the food cord and the question
should be invedigated further.
XIII. THE DORSAL STRAND
I n the adult uscidian the duct of the neural gland extends posteriorly
(t8 a fine strand of tissue, the dorsal strand or dorsal cord. A short
distance behind the gland it leaves the mantle and crosses to the dorsal
wall of the pharynx where it runs close above the dorsal lamina.
Posterior to thcb branchial sac in enterogonous aacidians (i.e. Ciona) it
extends to the gonad where it penetrates into the ovary and terminates
abruptly as a slight swelling (Millar, 1963s; Aubert, 1954). In pleurogonous ascidiaiis in which the gonads are in the mantle wall (i.e.
Molgula) the strand terminates abruptly in the neighbourhood of the
digestive diverticultl of the stomach (Van Beneden and Julin, 1884).
It is gonerally tigreed that this strand is derived from that part of the
larval neural tube which also gives rise to the neural gland and ganglion
(Willey, 1893 ; Van Beneden and Julin, 1884 ; HUUS, 1924 ; Brien, 1927 ;
Berrill. 1960; Millar, 1953a; Aubert, 1964).
Although the strand is derived from embryonic neural tissue there
are no nerve cells or fibres in it. In Molgula manhatten& Van Beneden
and Julin (1884) described the strand as composed of large cells resembling the large cells of the cerebral ganglion and arrangedradially around
a central core or small cavity. However, these authors pointed out that
the cells in M. rnrsnhattensia were larger than in many other species
examined, and Millar (l963a) describes the cells in Ciona intestinalis as
“ quite small, and spindle shaped, being somewhat elongated along the
strand ”. In Ciona the whole strand ie only 10 p to 16 p in diameter.
Huus (1924) does not figure the cord in section but his illut3trations
show rounded cells scattered unevenly in the strand of Corella paralldogramma (Muller). Brien (1927) describes them in the following term8
for Aplidium, pallidurn : ‘‘ Lea cellulen sont itlentiques 8, celles du canal
t’xcrdteiir, ou plug extlctement aux amoeboc*yten clu sang avec lesquek