164 
THE VOYAGE OF H.M.S. CHALLENGER. 
their transverse direction separates the group from Polyclinoides, Amaroucium, and 
Sigillina (see Table, p. 152) ; but in general appearance, and especially in size of Ascidio- 
zooid, Atopogaster aurantiaca and Atopogaster elongata differ so remarkably that I am 
inclined to think that they ought to belong to distinct sections of the family. Then in some 
respects Atopogaster aurantiaca appears to be allied to Tylohranclnion speciosum, from 
which, however, it differs totally in the condition of the branchial sac. Atopogaster gigantea, 
again, differs from the other members of the genus in the simple and conspicuous systems 
formed by the Ascidiozooids (see PI. XXIII. fig. 2), and in this respect approaches some 
species of Polyclinum (see Polyclinum pyriformis). The relationships of the different 
Polyclinidae are very complicated. They seem to form an irregular network, and it is 
very diffi.cult to determine which characters indicate most clearly the genetic affinity. 
In aU the species of this genus, however, the colony is massive, the amount of test 
present is large, and it is tough and cartilaginous in its nature ; the Ascidiozooids are 
greatly elongated antero-posteriorly, and the post-abdomen is large ; and lastly, in all, 
the stomach is more or less distinctly folded in a transverse direction. 
The species may be distinguished by means of the following table : — 
Atopogaster. 
Ascidiozooids arranged in distinct Systems inconspicuous, 
circular systems. | 
. -L .. I ^ Ti 
A. gigantea. Ascidiozooids large, more Ascidiozooids small, less 
than 1 cm. in length. than 1 cm. in length. 
^ I 
I I A. elongata, and 
Colour orange. Colour light grey. 4. elongata var. pallida. 
1 . I 
A. aurantiaca, A. informis. 
This arrangement divides the genus into three sections which seem to be natural : — 
(1) With distinct systems, Atopogaster gigantea; (2) with very large Ascidiozooids, 
Atopogaster aurantiaca and Atopogaster informis ; and (3) with small Ascidiozooids, 
Atopogaster elongata, and Atopogaster elongata var. pallida. 
Atopogaster gigantea, n. sp. (PI. XXIII. figs. 1-6). 
The Colony is a very large mass of elongated form attached by the posterior end, and 
extending upwards with a slight curve to a considerable height. It narrows gradually 
above the middle, and the upper end is obtusely pointed. The colony is slightly com- 
pressed laterally. The colour is an opaque whitish-grey with a slight hyaline tinge in 
places. The surface is somewhat uneven, and is slightly rough all over. 
