KEPORT ON THE TXJNICATA. 
383 
A new geniTS, Symplegma (p. 144), lias been formed for a remarkable colony obtained 
at Bermuda. This form differs from all the other Distomidse in having internal 
longitudinal bars in the branchial sac, but it is really very doubtful whether the genus 
ought to be placed in this family. I am rather inclined to suspect that Sijmplegma wall 
turn out to be an aberrant form allied to the Botryllidae (see p. 397). 
The large family Polyclinidse is represented in the Challenger collection by forty-three 
species and at least three w'eU-marked varieties. One species {Aplidium fallax) was 
previously known, the rest are all new to science. Ten genera are represented, and five 
of these {Pharyngodictyon, Tylohrancliion, Atopogaster, Morchellioides, and Psamma- 
plidium) are new groups (see p. 151) formed for the reception of Challenger species. 
The most remarkable form in the family is undoubtedly Pharyngodictyon mirahile 
(see p. 153), in which the branchial sac is in the curious degenerate condition found in 
Cideolus amongst Simple Ascidians. The internal longitudinal bars and the transverse 
vessels form a square-meshed network, which is not broken up into stigmata, as the 
system of fine longitudinal vessels seems to be entirely absent. This peculiar condition of 
the branchial sac seems to be associated with the abyssal zone, as it has apparently been 
evolved independently in at least four different groups of deep-sea Ascidians, viz., 
Culeolus, Fungulus, Bathyoncus, and Pharyngodictyon, while it has not been found in 
any forms from shallow water. 
In Pharyngodictyon then, internal longitudinal bars are present in the branchial sac, 
while they are absent in aU the other Polyclinidse. In Tylohranchion speciosum, however, 
the transverse vessels bear curious papillae (see PI. XXII. fig. 7), which are probably 
rudimentary connecting ducts which have lost their proper function, as there are no internal 
longitudinal bars present to support them, but which have not yet disappeared. The 
branchial sac of Tylohranchion speciosum is therefore in an intermediate condition of 
degeneration between that of Pharyngodictyon mirahile and that of the other Polyclinidse 
(see phylogeny of the group, p. 390). 
The genus Atopogaster includes some new species of large size which have the 
stomach-wall more or less transversely folded, an unusual condition in Ascidians. 
The curious group of species characterised by that irregularly thickened condition of 
the stomach-wall called areolated by Giard, is represented in the collection by three forms 
aU new to science, viz., Morchellioides affinis, Morchellium giardi, and Sidnyum pallidum. 
The first of them (see p. 177) has eight lobes around the branchial aperture, while in most 
of the Compound Ascidians the branchial apertures are always six-lobed. 
The remaining new genus, Psammaplidium, is closely allied to Aplidium, but has the 
test strengthened by imbedded sand-grains (see p. 237 and PI. XXXI. fig. 9). It contains 
nine new species, all from the southern hemisphere. 
The Polyclinidse as a family are very widely distributed, but the majority of the 
Challenger species are from the far south. 
