The nature of the ascidian tadpole, with reference to Boltenia echinata

Abstract

FOUR FIGURXSOver a period of more than 2 decades a variety of ascidian tadpoles have been described by Caswell Grave, always with meticulous accuracy. The last paper, published posthumously ('44) and introduced by the late S. 0. Mast, concerned the tadpole of Styela partita, specifically confirming and extending an earlier account by Berrill ('29) that the larval eye was greatly reduced in relative size. Many unusual statements are made in this paper which cannot be adequately discussed here. Essentially, Grave sees such divergence in larval characters, with particular reference to larval sense organs, that he regards the larvae of species with ' ' eyes characterized by a pigment-cup, photosensitive cells and systems of lenses" as having an origin independent of, for example, those of Rtyela, Molgula and Botryllus; that "studies of the larval forms of ascidians show a diversity in structure and activities greater than is found in the adults" and he states that ascidian systematists regard the larva of ascidians as the recapitulated ancestral form of the group and fail to give it proper diagnostic systematic value.I n way of general response it should be said first, that ascidian systematists (Van Name, '21 ; Garstang, '28 ; arid Eerrill, '31, '45) have f o r many years held the view finally stated by Grave, that the ancestral ascidian was a sessile form and the tadpole larva an ascidian creation, and secondly, that the nature of the larva has been given extensive diagnostic 263

Statocyte and Ocellar Pigment Cell in Embryos and Larvae of the Ascidian, Styela plicata (Lesueur)

“…In all species of Molugulidae, the ocellus has completely degenrated and in some of them the statocyte is also lost. In these species, the statocyte does not differentiate into a photolith (4).…”

“…According to Berrill (4), larvae of Enterogona and Pyuridae (Pleurogona) have both a statocyte and a typical complex ocellus (Fig. 5, A).…”

“…5, B), the ocellus is completely degenerated and the statocyte forms a specialized structure, the photolith: that is, a pigment cup containing photo-sensitive processes of retinal cells connected with a ganglion (1, 3, 4, 5, In the larval cerebral vesicle of some species of Styelinae (e.g. Polycarpa rustica) there is a statocyte, but no ocellus (4). In all species of Molugulidae, the ocellus has completely degenrated and in some of them the statocyte is also lost.…”

“…Ciona (6,8,9,14,21,22) and Arnarouciurn (2), and of Pyuridae (Pleurogona), e.g. Boltenia (4), is cup-shaped and contains many small pigment granules. In these species, a typical ocellus is also furnished with three lens cells in front of the pigment cup, and several retinal cells protrude photo-sensitive processes into the cup.…”

“…In the larvae of Botryllinae, the ocellus has been completely lost and the statocyte has become a specialized organ, the "photolith", that is sensitive to both light and gravity (1,3,4,5,11,12). The larvae of Styelinae have a statocyte and an ocellus, but the ocellus consists of only a single cell containing a tiny mass of pigment granules in the postero-dorsal wall of the cerebral vesicle in Styela yakatutensis (4), S. partifa (10, 21,22) and S. rustica (15). The larva of Styela plicata, investigated in this work, has two pigment cells, a large statocyte and a small degenerated ocellar pigment cell (13).…”