16
It. H. BlILLAR
observed the larvae of Peropbra viridis (Verrill) being shed from
colonies in the early morning, and there may be periodicity also in
Botryllus schlosseri (Grave and Woodbridge, 1924). In Molgula citrina
(Alder and Hancock), however, no definite period of release has been
observed, under laboratory conditions (Grave, 1926), nor in Metandrocarpa taylori (Abbott, 1955).
2. Structure
Throughout the group many structural variations have appeared in
the larva, as illustrated in Fig. 5 which shows the principal forms now
known. We still have little idea, however, of the functional significance
of the different kinds of adhesive papillae, anterior ampullae and epidermal vesicles which vary widely and have been used in interpreting
the phylogeny of ascidians (Kott, 1969). The simplest type of larva,
which probably represents the ancestral form, has an ovoid trunk with
three conical adhesive papillae in a triangular arrangement, and no
ampullae or epidermal vesicles. This form occurs in the Cionidae,
Diazonidae, Ascidiidae, Corellidae, Pyuridae, Molgulidae and most of
the solitary members of the Styelidae, and the larva is generally small,
with a trunk from 0-15-0-30 mm in length. It is amongst the compound
ascidians (families Clavelinidae, Polyclinidae, Didemnidae and subfamily Botryllinae) that the greatest modifications in larval structure
have appeared. Here the papillae are usually borne on stalks, and often
have a terminal cup with a central cone of secretory cells, but notable
exceptions are the invaginated tubular papillae of Euherdmania
(Trason, 1957; Millar, 1961a) (Fig. 5, no. 10) and Pycnoclavelh (Berrill,
1950; Trason, 1963) (Fig. 5, no. 2) and the narrow elongated structures
of Eudistoma fantasianum (Kott, 1957a) and E. digitatum (Millar,
1964a) (Fig. 5, no. 8).
The dual nature of the ascidian tadpole (Grave, 1935 ; Berrill, 1955 ;
Millar, I966b), which serves the larval purposes of distribution and site
selection and also carries the rudiments of the adult, has profoundly
affected larval structure. Thus, amongst the compound forms, the
rudimentary adult may already show small buds, or differentiated
blastozooids as in Diplosoma (Fig. 5 , no. 17) and Polysyncraton magnilarvum (Millar, 196213) (Fig. 5, no. 16), or a sufficient set of blastozooids
to constitute a small colony shortly after larval attachment, as in
Hypsistozoa fasmerianu (Brewin, 1956a, 1959) (Fig. 5, no. 4). Larval
size, too, is greatest amongst the compound forms. In many of these
the larval trunk is 0.5-1.0 mm in length-considerably greater than the
average size amongst the simple forms-and in a few it is much larger.
Amongst the giants are the larvae of Polysyncraton magnilarvum at
It. H. BlILLAR
observed the larvae of Peropbra viridis (Verrill) being shed from
colonies in the early morning, and there may be periodicity also in
Botryllus schlosseri (Grave and Woodbridge, 1924). In Molgula citrina
(Alder and Hancock), however, no definite period of release has been
observed, under laboratory conditions (Grave, 1926), nor in Metandrocarpa taylori (Abbott, 1955).
2. Structure
Throughout the group many structural variations have appeared in
the larva, as illustrated in Fig. 5 which shows the principal forms now
known. We still have little idea, however, of the functional significance
of the different kinds of adhesive papillae, anterior ampullae and epidermal vesicles which vary widely and have been used in interpreting
the phylogeny of ascidians (Kott, 1969). The simplest type of larva,
which probably represents the ancestral form, has an ovoid trunk with
three conical adhesive papillae in a triangular arrangement, and no
ampullae or epidermal vesicles. This form occurs in the Cionidae,
Diazonidae, Ascidiidae, Corellidae, Pyuridae, Molgulidae and most of
the solitary members of the Styelidae, and the larva is generally small,
with a trunk from 0-15-0-30 mm in length. It is amongst the compound
ascidians (families Clavelinidae, Polyclinidae, Didemnidae and subfamily Botryllinae) that the greatest modifications in larval structure
have appeared. Here the papillae are usually borne on stalks, and often
have a terminal cup with a central cone of secretory cells, but notable
exceptions are the invaginated tubular papillae of Euherdmania
(Trason, 1957; Millar, 1961a) (Fig. 5, no. 10) and Pycnoclavelh (Berrill,
1950; Trason, 1963) (Fig. 5, no. 2) and the narrow elongated structures
of Eudistoma fantasianum (Kott, 1957a) and E. digitatum (Millar,
1964a) (Fig. 5, no. 8).
The dual nature of the ascidian tadpole (Grave, 1935 ; Berrill, 1955 ;
Millar, I966b), which serves the larval purposes of distribution and site
selection and also carries the rudiments of the adult, has profoundly
affected larval structure. Thus, amongst the compound forms, the
rudimentary adult may already show small buds, or differentiated
blastozooids as in Diplosoma (Fig. 5 , no. 17) and Polysyncraton magnilarvum (Millar, 196213) (Fig. 5, no. 16), or a sufficient set of blastozooids
to constitute a small colony shortly after larval attachment, as in
Hypsistozoa fasmerianu (Brewin, 1956a, 1959) (Fig. 5, no. 4). Larval
size, too, is greatest amongst the compound forms. In many of these
the larval trunk is 0.5-1.0 mm in length-considerably greater than the
average size amongst the simple forms-and in a few it is much larger.
Amongst the giants are the larvae of Polysyncraton magnilarvum at