14
R. € I .
a functional relationship exists between the neural complex (ganglion
and neural gland) and sexual activity and this work has been summarized by Dodd (1955) and by Hisaw et al. (1966). Despite conflicting
experimental results and the discovery of neurosecretory cells in the
ganglion (Dawson and Hisaw, 1964), it is at least doubtful if the neural
complex is essential to the act of spawning, for Hisaw et al. noted
normal gonad development and discharge of gametes in animals deprived of the complex for periods of up to a year. Nevertheless, the
experiments of Sengel and Kieny (1962)) Sengel and Georges (1966) and
Bouchard-Madrelle (1967) all strongly suggest that the neural complex
has some influence on the development of the gonads and on spawning.
C. The larva
1. Development and release
Most solitary forms release their gametes into the sea, where
fertilization and development take place, but almost all compound
ascidians retain their eggs until the larva is complete and able to swim.
A number of ways have been adopted of protecting the embryos during
development, by retaining them within the oviduct, the atrial cavity or
a brood pouch of the zooid, or in the test matrix of the colony. The most
elaborate method yet discovered is in the New Zealand species Hypsist o m fasmerianu (Michaelsen) (Brewin, 1956a). In this species the ovary
produces a single egg, only 25 p in diameter, which develops into a
large larva in an oviducal brood pouch, there receiving nourishment
through a pair of larval endodermal tubes. During the whole developmental period of 5& months, attachment to the parental zooid is maintained, and the resulting larva is very complex, with numerous buds.
The advanced stage of development attained by the larva before release
must be of considerable advantage in founding the new generation. A
similar objective is achieved, in quite a different way, by the solitary
Polycurpa tinctor (Quoy and Gaimard). Here the egg is very large
(730 p in diameter) and rich in yolk, and develops within the atrial
chamber directly into a miniature ascidian, without the intervention of
a larval stage (Millar, 19628,).
Larvae escape from the parent colony in various ways, according t o
the site of incubation. In most species the developing embryos are
retained in the thorax and the larvae pass out directly through the
atrial siphon where this opens on the surface of the colony, or via the
common cloaca1 cavities. In Euherdrnania claviformis Trason (1957)
observed the passage of mature embryos from the oviduct to the
atrium of the zooid, a process taking about 10 minutes, while the passage
through the atrium lasted 3 4 5 0 minutes. Apparently the immature
R. € I .
a functional relationship exists between the neural complex (ganglion
and neural gland) and sexual activity and this work has been summarized by Dodd (1955) and by Hisaw et al. (1966). Despite conflicting
experimental results and the discovery of neurosecretory cells in the
ganglion (Dawson and Hisaw, 1964), it is at least doubtful if the neural
complex is essential to the act of spawning, for Hisaw et al. noted
normal gonad development and discharge of gametes in animals deprived of the complex for periods of up to a year. Nevertheless, the
experiments of Sengel and Kieny (1962)) Sengel and Georges (1966) and
Bouchard-Madrelle (1967) all strongly suggest that the neural complex
has some influence on the development of the gonads and on spawning.
C. The larva
1. Development and release
Most solitary forms release their gametes into the sea, where
fertilization and development take place, but almost all compound
ascidians retain their eggs until the larva is complete and able to swim.
A number of ways have been adopted of protecting the embryos during
development, by retaining them within the oviduct, the atrial cavity or
a brood pouch of the zooid, or in the test matrix of the colony. The most
elaborate method yet discovered is in the New Zealand species Hypsist o m fasmerianu (Michaelsen) (Brewin, 1956a). In this species the ovary
produces a single egg, only 25 p in diameter, which develops into a
large larva in an oviducal brood pouch, there receiving nourishment
through a pair of larval endodermal tubes. During the whole developmental period of 5& months, attachment to the parental zooid is maintained, and the resulting larva is very complex, with numerous buds.
The advanced stage of development attained by the larva before release
must be of considerable advantage in founding the new generation. A
similar objective is achieved, in quite a different way, by the solitary
Polycurpa tinctor (Quoy and Gaimard). Here the egg is very large
(730 p in diameter) and rich in yolk, and develops within the atrial
chamber directly into a miniature ascidian, without the intervention of
a larval stage (Millar, 19628,).
Larvae escape from the parent colony in various ways, according t o
the site of incubation. In most species the developing embryos are
retained in the thorax and the larvae pass out directly through the
atrial siphon where this opens on the surface of the colony, or via the
common cloaca1 cavities. In Euherdrnania claviformis Trason (1957)
observed the passage of mature embryos from the oviduct to the
atrium of the zooid, a process taking about 10 minutes, while the passage
through the atrium lasted 3 4 5 0 minutes. Apparently the immature