96
IVAN m o m o m
cavity is also c.oncwiitd with maintenaiice of water pressure and the
speed with which wstcr flows out through the atrial siphon. It is
possible that t lie cupulih organs ;Ire concerned with this activity (Fedele,
10231~).
The occ~lli w e pigmented sfmts found in many, if not all, ascidiaiis in
the notches between the lobes of the siphons (Fig. 26). They are very
conspicuous in Cionn intestinalis and in the very young stages of species
of Ascidia but in the latter may later be masked by the development of
other pigment. Descriptions of these structures have been given by
several authors including 8eeliger and Hartmeyer (1893-191 l), Haffner
(1933), Huus (1937), Brien (1948) and Millar (1953a). Each ocellus
consists of a small depression lined by sensory cells and the whole
backed by an accumulation of pigment granules. Haffner describes
minute hair processes arising from the apex of each cell but Nillar was
unable to detect them. Seeliger also illustrates such processes but arising
from the cell boundaries. Neither Millar nor Scudder and Karczmar
(1966) were able to det,cxt any nervous innervation of the ocelli but
Markman (1!)88) reported fibres extending from them back to the
ganglion. Thv ocelli tire further discussed in the next section.
I). Photoreceplion
The respoiises of uscidians to light were studied by Hecht (1918b,
1918d, 1926) in A,scidia nigra and Ciona intestinulis and no further study
of significance st:cms to have appeared since that time. In Ciona Hecht
showed quite c*onclu.sively that the " ocelli " are not light receptors and
therefore must have some other function. The name '' ocellus " leads
to confusion about thcir Sunction and it would be appropriate to find a
new terminology, but perhaps this should wait until their true role is
made certain. Ciona does, however, show two very dintinctive typen of
response to light, one producing a psiti ve phototropinm, the othrr
producing tb shiirp r&action of the body or " escape '' reaction.
Hecht (1!)18d) showed that, the posterior visceral region of Ciona is
t>otally insensitive to light, the tmterior tip is only sensitive in the region
of the gunglioii, and the whole branchial region is highly sensitive. Tf
the brunc.hittl region is illuminsted on 011e side the mantle musculaturc
of that side will contract so that the animal bends towards the illuminated side. Millar (1!)53a) suggested that the ocelli may play a role in
this phototropic response but in doing so seems to have overlooked
Hecht's very ronvincing studies. Hecht was further able to show that
the plmtotropicl rcsponse is independent of the cerebral ganglion and
unaffected by its removttl, rind he suggested that purely local pathways.
perhaps tjlirougli a ntwe net, must be involved. There are no specialized
IVAN m o m o m
cavity is also c.oncwiitd with maintenaiice of water pressure and the
speed with which wstcr flows out through the atrial siphon. It is
possible that t lie cupulih organs ;Ire concerned with this activity (Fedele,
10231~).
The occ~lli w e pigmented sfmts found in many, if not all, ascidiaiis in
the notches between the lobes of the siphons (Fig. 26). They are very
conspicuous in Cionn intestinalis and in the very young stages of species
of Ascidia but in the latter may later be masked by the development of
other pigment. Descriptions of these structures have been given by
several authors including 8eeliger and Hartmeyer (1893-191 l), Haffner
(1933), Huus (1937), Brien (1948) and Millar (1953a). Each ocellus
consists of a small depression lined by sensory cells and the whole
backed by an accumulation of pigment granules. Haffner describes
minute hair processes arising from the apex of each cell but Nillar was
unable to detect them. Seeliger also illustrates such processes but arising
from the cell boundaries. Neither Millar nor Scudder and Karczmar
(1966) were able to det,cxt any nervous innervation of the ocelli but
Markman (1!)88) reported fibres extending from them back to the
ganglion. Thv ocelli tire further discussed in the next section.
I). Photoreceplion
The respoiises of uscidians to light were studied by Hecht (1918b,
1918d, 1926) in A,scidia nigra and Ciona intestinulis and no further study
of significance st:cms to have appeared since that time. In Ciona Hecht
showed quite c*onclu.sively that the " ocelli " are not light receptors and
therefore must have some other function. The name '' ocellus " leads
to confusion about thcir Sunction and it would be appropriate to find a
new terminology, but perhaps this should wait until their true role is
made certain. Ciona does, however, show two very dintinctive typen of
response to light, one producing a psiti ve phototropinm, the othrr
producing tb shiirp r&action of the body or " escape '' reaction.
Hecht (1!)18d) showed that, the posterior visceral region of Ciona is
t>otally insensitive to light, the tmterior tip is only sensitive in the region
of the gunglioii, and the whole branchial region is highly sensitive. Tf
the brunc.hittl region is illuminsted on 011e side the mantle musculaturc
of that side will contract so that the animal bends towards the illuminated side. Millar (1!)53a) suggested that the ocelli may play a role in
this phototropic response but in doing so seems to have overlooked
Hecht's very ronvincing studies. Hecht was further able to show that
the plmtotropicl rcsponse is independent of the cerebral ganglion and
unaffected by its removttl, rind he suggested that purely local pathways.
perhaps tjlirougli a ntwe net, must be involved. There are no specialized