30
It. H. ItULLAR
winter. Similarly Archidistoma aggregatum and Diazona violaeea
Savigny develop during the winter (Berrill, 1948b, 1948~). In functional
colonies of Metandrocarpa taylori Huntsman asexual reproduction takes
place throughout the year (Abbott, 1953), but is least intensive when
breeding is in progress, in the summer (Haven, 1967). Sabbadin (1955)
recorded budding in Botryllus schlosseri at all seasons, although the
process was slower in winter.
Naukauchi (1966a) considered that budding is of two functional
types. One of these involves resorption of the zooid, is an adaptation to
adverse conditions, and may take the form of hibernation (Clavelina,
Diazona, Syndiazona, Perophora) or aestivation (Aplidium multiplicatum) ; the other, which is not accompanied by resorption of the
zooid, occurs in favourable conditions and results in growth of the
colony (Perophora, botryllids, polycitorines and some polyclinids). The
alternation of the sexual and asexual process in compound ascidians is
therefore a complex pattern of events, further complicated by the fact
that in some genera all buds can reproduce both sexually and asexually
whilst in others different sets of buds are restricted to one method
(Ivanova-Kazas, 1967).
A few compound ascidians, in which the colony is very distinctly
divided into stalk and head, show particularly clearly a cyclic degeneration and regeneration. Sycozoa sigillinoides Lesson has a long,
narrow, sometimes branched stalk surmounted by heads containing the
functional zooids. Following breeding, the heads disintegrate or break
off, leaving headless stalks containing buds derived from the vascular
processes of zooids (Caullery, 1909; Salfi, 1925) (Fig. 14). In this
reduced state the stalks recall the overwintering condition of certain
species of Polyclinidae. The stalks of Sywzoa subsequently grow new
heads, when the buds develop into zooids which secrete fresh test
material (Millar, 1960). It would be interesting to know whether the
whole process is repeated year after year. In the related genus Hgpsistozoa, one of the most highly evolved of all compound ascidians, having
a very complex embryonic and larval development (Brewin, 1956a), the
colony is also divided into a stalk and head, and, as in Sycozoa, breeding
is followed by dissolution of the head which is later replaced through the
activity of zooids at the top of the stalk. Hypsistozoa illustrates the
alternation of sexual and asexual processes (Fig. 15), and it is interesting to note that budding and growth of the colony occur in summer
and autumn whereas sexual reproduction is confined to the winter and
spring, thus reversing the more usual seasonal sequence. A plentiful
supply of plankton is available during gonad maturation and embryological development (Brewin, 1956a), and may be particularly im-
It. H. ItULLAR
winter. Similarly Archidistoma aggregatum and Diazona violaeea
Savigny develop during the winter (Berrill, 1948b, 1948~). In functional
colonies of Metandrocarpa taylori Huntsman asexual reproduction takes
place throughout the year (Abbott, 1953), but is least intensive when
breeding is in progress, in the summer (Haven, 1967). Sabbadin (1955)
recorded budding in Botryllus schlosseri at all seasons, although the
process was slower in winter.
Naukauchi (1966a) considered that budding is of two functional
types. One of these involves resorption of the zooid, is an adaptation to
adverse conditions, and may take the form of hibernation (Clavelina,
Diazona, Syndiazona, Perophora) or aestivation (Aplidium multiplicatum) ; the other, which is not accompanied by resorption of the
zooid, occurs in favourable conditions and results in growth of the
colony (Perophora, botryllids, polycitorines and some polyclinids). The
alternation of the sexual and asexual process in compound ascidians is
therefore a complex pattern of events, further complicated by the fact
that in some genera all buds can reproduce both sexually and asexually
whilst in others different sets of buds are restricted to one method
(Ivanova-Kazas, 1967).
A few compound ascidians, in which the colony is very distinctly
divided into stalk and head, show particularly clearly a cyclic degeneration and regeneration. Sycozoa sigillinoides Lesson has a long,
narrow, sometimes branched stalk surmounted by heads containing the
functional zooids. Following breeding, the heads disintegrate or break
off, leaving headless stalks containing buds derived from the vascular
processes of zooids (Caullery, 1909; Salfi, 1925) (Fig. 14). In this
reduced state the stalks recall the overwintering condition of certain
species of Polyclinidae. The stalks of Sywzoa subsequently grow new
heads, when the buds develop into zooids which secrete fresh test
material (Millar, 1960). It would be interesting to know whether the
whole process is repeated year after year. In the related genus Hgpsistozoa, one of the most highly evolved of all compound ascidians, having
a very complex embryonic and larval development (Brewin, 1956a), the
colony is also divided into a stalk and head, and, as in Sycozoa, breeding
is followed by dissolution of the head which is later replaced through the
activity of zooids at the top of the stalk. Hypsistozoa illustrates the
alternation of sexual and asexual processes (Fig. 15), and it is interesting to note that budding and growth of the colony occur in summer
and autumn whereas sexual reproduction is confined to the winter and
spring, thus reversing the more usual seasonal sequence. A plentiful
supply of plankton is available during gonad maturation and embryological development (Brewin, 1956a), and may be particularly im-