the dorsal strand should be named the " vimeral $fibre complex " to
distinguish it from the visceral nerve. The dorsal strand is not an
intimate part of this visceral system but may nourish it or may continue
to provide new nerve cells as the rinimul grows (p. 125). The visceral
innervation to the branchial siw and to the alimentary canal is derived
from the visceral fibre complex and in both cases forms a non-polarized
network which controls d i a r y activity. Other viseeral nervous elements
also enter the branchial sac particularly through the endostyle, and the
alimentary curial may also have a secoiid innervation. The endostyle is
also innervated by anterior nerves arising from the ganglion but these
are probably Recrcto-motor in function. The heart muscle has no
innervation and is entirely niyogcmic b u t the pericardium has a fine
nerve plexus, possibly part of a sensory-motor reflex.
C. The sensory syatem
The sessile rnode of life adopted by ascidians makes only relatively
small demands upon the sensory system. There are no problems of
locomotion and exploratory activity and almost all of the sensory
system and its motor responses are concerned with protection or defence.
Once metamorphosis is complete the animal has no further choice in
selection of tho environment and food supply available to it and becomes
adapted toward either accepting or rejecting what is presented. The
only motor responses availablct to the animal are to expand the mantle
wall and siphons and thereby accept the cmvironment, or to contract the
mmtle and close the siphons thereby rejecting the environment. For a
few ascidians such as C i o m thc whole body can aontract longitudinally
thus providing a limited " eswipe " reaction, but for the majority the
rigid limitations of the test rcstrict their activity primarily to mantle
contraction and only limited shortening of the body. Within the framework of these limitations it is to be expected that rnechanoreceptors
and chemoreceptors should predominate and that photoreceptors might
also be present. Sensory cells and afferent fibres have been described by
a number of authors but until more precise electrophysiological work
can be carried ont the exact functions of some of these must remain in
doubt.
Hunter (1898) described cells in the endostyle and pericardium
which he believed were sensory. These are figured as simple bipolar
cells scattered throughout the endostyle and in the raphe of the pericardium. Lorleberg (1 907) believed that' there were no sense cell8 in
ugcidians but that free nerve endings existed scattered throughout thcb
body wull. 1)us (I948b) also rlaimed tha existcnce of' such free nerve
endings throughout the body wtill although elsewhere (Urn, 1936) he
distinguish it from the visceral nerve. The dorsal strand is not an
intimate part of this visceral system but may nourish it or may continue
to provide new nerve cells as the rinimul grows (p. 125). The visceral
innervation to the branchial siw and to the alimentary canal is derived
from the visceral fibre complex and in both cases forms a non-polarized
network which controls d i a r y activity. Other viseeral nervous elements
also enter the branchial sac particularly through the endostyle, and the
alimentary curial may also have a secoiid innervation. The endostyle is
also innervated by anterior nerves arising from the ganglion but these
are probably Recrcto-motor in function. The heart muscle has no
innervation and is entirely niyogcmic b u t the pericardium has a fine
nerve plexus, possibly part of a sensory-motor reflex.
C. The sensory syatem
The sessile rnode of life adopted by ascidians makes only relatively
small demands upon the sensory system. There are no problems of
locomotion and exploratory activity and almost all of the sensory
system and its motor responses are concerned with protection or defence.
Once metamorphosis is complete the animal has no further choice in
selection of tho environment and food supply available to it and becomes
adapted toward either accepting or rejecting what is presented. The
only motor responses availablct to the animal are to expand the mantle
wall and siphons and thereby accept the cmvironment, or to contract the
mmtle and close the siphons thereby rejecting the environment. For a
few ascidians such as C i o m thc whole body can aontract longitudinally
thus providing a limited " eswipe " reaction, but for the majority the
rigid limitations of the test rcstrict their activity primarily to mantle
contraction and only limited shortening of the body. Within the framework of these limitations it is to be expected that rnechanoreceptors
and chemoreceptors should predominate and that photoreceptors might
also be present. Sensory cells and afferent fibres have been described by
a number of authors but until more precise electrophysiological work
can be carried ont the exact functions of some of these must remain in
doubt.
Hunter (1898) described cells in the endostyle and pericardium
which he believed were sensory. These are figured as simple bipolar
cells scattered throughout the endostyle and in the raphe of the pericardium. Lorleberg (1 907) believed that' there were no sense cell8 in
ugcidians but that free nerve endings existed scattered throughout thcb
body wull. 1)us (I948b) also rlaimed tha existcnce of' such free nerve
endings throughout the body wtill although elsewhere (Urn, 1936) he