XIV. CONVLIJSION
The foregoing pages illustrate a number of interesting problems in
asciditrn physiology, inany of them still incompletely resolved. Recent
work on the test haa revealed the detail of fine structure and the intervention of morula cells in the deposition of complex polysaccharide
microfibrils. We still do not understand the precise role of the morela
cell in this process and why some ascidians have found it necessary to
use vanadium and others iron in the organo-metal complex of the cell.
Different groups of ascidians have tests of different organization varying
from gelatinous to fibrous and a comparative study of the architecture
of the test and its functional significance would be of interest.
Studies on the endostylc over the past ten years have elucidated the
fine structure arid function of this organ and the type of secretion which
it produces. We are beginning to have an understanding of the functional significance of differences in branchial organization and how it
relates to food collection, but there is still much work to be done in this
field. In particular the role of the papillae in food transport is uncertain
and little is known about the mode of functioning of the dorsal lamina
and why its organization differs between species. My own work in
Jamaica suggests that different ascidians are exploiting different
fractions of the food supply available in the environment, but how this
is achieved is unknown. There are scattered references in the literature
to the possibility of food sorting and selection by the branchial sac, but
this has never been confirmed.
There has been little advance in our understanding of digestive
physiology although recent studies with electron microscopy have
revealed new detail about the nature of the alimentary canal. Some
doubt is expressed in this review about the accepted interpretation of
food transport and treatment and the role of the pyloric gland; this
aspect of ascidian physiology requires new and critical examination.
There are also many unresolved problems of metabolism in aacidians
particularly as to whether the neural gland contributes any enzymatic
material to the food cord and how metabolism proceedu beyond gaRtric
digestion. Recent studies have confirmed earlier suggestions that
ascidians are entirely ammonotelic in their nitrogen metabolism but
store products of nucleic acid metabolism as urates or higher purines.
However, we still do not know whether such storage is a biochemical
accident or whether i t has particular funct iond significance.
Considerable advances have been made in recent years in the field
of cardiovascular physiology of ascidians. The myogenic nature of the
heart! and its control by terminal pacemakers is well established as are
The foregoing pages illustrate a number of interesting problems in
asciditrn physiology, inany of them still incompletely resolved. Recent
work on the test haa revealed the detail of fine structure and the intervention of morula cells in the deposition of complex polysaccharide
microfibrils. We still do not understand the precise role of the morela
cell in this process and why some ascidians have found it necessary to
use vanadium and others iron in the organo-metal complex of the cell.
Different groups of ascidians have tests of different organization varying
from gelatinous to fibrous and a comparative study of the architecture
of the test and its functional significance would be of interest.
Studies on the endostylc over the past ten years have elucidated the
fine structure arid function of this organ and the type of secretion which
it produces. We are beginning to have an understanding of the functional significance of differences in branchial organization and how it
relates to food collection, but there is still much work to be done in this
field. In particular the role of the papillae in food transport is uncertain
and little is known about the mode of functioning of the dorsal lamina
and why its organization differs between species. My own work in
Jamaica suggests that different ascidians are exploiting different
fractions of the food supply available in the environment, but how this
is achieved is unknown. There are scattered references in the literature
to the possibility of food sorting and selection by the branchial sac, but
this has never been confirmed.
There has been little advance in our understanding of digestive
physiology although recent studies with electron microscopy have
revealed new detail about the nature of the alimentary canal. Some
doubt is expressed in this review about the accepted interpretation of
food transport and treatment and the role of the pyloric gland; this
aspect of ascidian physiology requires new and critical examination.
There are also many unresolved problems of metabolism in aacidians
particularly as to whether the neural gland contributes any enzymatic
material to the food cord and how metabolism proceedu beyond gaRtric
digestion. Recent studies have confirmed earlier suggestions that
ascidians are entirely ammonotelic in their nitrogen metabolism but
store products of nucleic acid metabolism as urates or higher purines.
However, we still do not know whether such storage is a biochemical
accident or whether i t has particular funct iond significance.
Considerable advances have been made in recent years in the field
of cardiovascular physiology of ascidians. The myogenic nature of the
heart! and its control by terminal pacemakers is well established as are