98
IVAN QOODBODY
received. Pcrhaps the answcr is to be found in the retraction response
to sudden chmges in thv intensity of illumination. This response is not
very Rennitivcb but it is iiecessary to focus light on the ganglion on the
interior of the! siphon before it can be clicitcd. Possibly under natural
conditions it will only function to produce a shadow reflex if the maximum possible illumination is falling on the sensitive regions. Alternatively, ascidians respond through these mechanisms to long term
ohanges in solar or lunar illurnination, and therefore orient towards the
brighter centre of illumination.
Because of their structural similarity to the simple light receptors
of many other invertebrates the ocelli have often been assumed to be
photorcceptorx as well, but in fact there is no evidence at all to support
this assumption. Ilas (l948a) tneritions them as photoreceptors but
Hecht (191 sb), Millar (1963a) und Millott ( 1 957) all categorically deny
a response to light by these structurw, although Millar concedes that
they might play a role in the phototropic response exhibited by the
siphons of Cionu intestinalis. Fedele (1 923a) illso concluded, on purely
morphologienl grounds, that they could not be photoreoeptors. If the
ocolli are not photoreceptors and if they are innervated as suggested by
Markman then thcy must serve some other function. It is unlikely that
they art' " wstigial structures " as suggested by Huus (1937). They
might, however, be a specialized form of mechanoreceptor engaged in
monitoring siphond notivity. Situated in the notch between siphonaI
lobes they arc ideally situated to be subjected to mechanical deformation as t,he siphons close and opon.
R , The sortlatic motor system
Before considering the n:bturc of ascidian responee mcchanisms it is
necessary to examine the nature of the musculature and how it controls
the animal. I t is probable that Cionu represents the most primitive
type of uscitlian organizatioti (Berrill, 1955; Millar, 1966) and the
muscles in thiit genus may u l ~ o represent the basic type of system. In
Cionu thc viwcra lie poxterior to the branchial sac and there are two
well-defined opposing scats of body muscles. Six major longitudinal
bands run Srom the siphons to the posterior end of thc body and act au
body retractors and siphon retractors. These are opposed by a set of
much fiiicr iind more numerous circular muscles girdling the body
throughout its length and modified anteriorly to form sphincter muscles
around the siphons. The test in Cionu is soft and almost jelly-like in itA
consistency and can play little if any role in the mechanics of movement.
A similar type of musculature persists in most aplousobranch genera,
all of which form colonies of zooids, and where the mechanics of move-
IVAN QOODBODY
received. Pcrhaps the answcr is to be found in the retraction response
to sudden chmges in thv intensity of illumination. This response is not
very Rennitivcb but it is iiecessary to focus light on the ganglion on the
interior of the! siphon before it can be clicitcd. Possibly under natural
conditions it will only function to produce a shadow reflex if the maximum possible illumination is falling on the sensitive regions. Alternatively, ascidians respond through these mechanisms to long term
ohanges in solar or lunar illurnination, and therefore orient towards the
brighter centre of illumination.
Because of their structural similarity to the simple light receptors
of many other invertebrates the ocelli have often been assumed to be
photorcceptorx as well, but in fact there is no evidence at all to support
this assumption. Ilas (l948a) tneritions them as photoreceptors but
Hecht (191 sb), Millar (1963a) und Millott ( 1 957) all categorically deny
a response to light by these structurw, although Millar concedes that
they might play a role in the phototropic response exhibited by the
siphons of Cionu intestinalis. Fedele (1 923a) illso concluded, on purely
morphologienl grounds, that they could not be photoreoeptors. If the
ocolli are not photoreceptors and if they are innervated as suggested by
Markman then thcy must serve some other function. It is unlikely that
they art' " wstigial structures " as suggested by Huus (1937). They
might, however, be a specialized form of mechanoreceptor engaged in
monitoring siphond notivity. Situated in the notch between siphonaI
lobes they arc ideally situated to be subjected to mechanical deformation as t,he siphons close and opon.
R , The sortlatic motor system
Before considering the n:bturc of ascidian responee mcchanisms it is
necessary to examine the nature of the musculature and how it controls
the animal. I t is probable that Cionu represents the most primitive
type of uscitlian organizatioti (Berrill, 1955; Millar, 1966) and the
muscles in thiit genus may u l ~ o represent the basic type of system. In
Cionu thc viwcra lie poxterior to the branchial sac and there are two
well-defined opposing scats of body muscles. Six major longitudinal
bands run Srom the siphons to the posterior end of thc body and act au
body retractors and siphon retractors. These are opposed by a set of
much fiiicr iind more numerous circular muscles girdling the body
throughout its length and modified anteriorly to form sphincter muscles
around the siphons. The test in Cionu is soft and almost jelly-like in itA
consistency and can play little if any role in the mechanics of movement.
A similar type of musculature persists in most aplousobranch genera,
all of which form colonies of zooids, and where the mechanics of move-