28 
THE VOYAGE OF H.M.S. CHALLENGER. 
general thickness. In the measurements of an Ascidiozooid, the length is the entire 
antero-posterior extent from the branchial aperture to the posterior end of the body, ex- 
clusive of the vascular appendage when present, the breadth is the greatest dorso-ventral 
extent, and the thickness is the distance from the right to the left hand side of the body. 
ASCIDIJE COMPOSITAE. 
The Ascidias Compositae, from which I exclude the family Clavelinidse, may be 
defined as fixed Ascidians which reproduce by gemmation so as to form colonies in 
which the Ascidiozooids are imbedded in a common test or investing mass. 
This suborder, then, is readily distinguished from the Thaliacea and the Larvacea, and 
from the Ascidiae Salpiformes, all of which are free swimming ; but although a typical 
Compound Ascidian may readily be separated by well-marked characteristics from a 
typical Simple Ascidian, it is very difficult if not impossible to frame a definition which 
will hold good for all the Ascidiae Compositae, while excluding aU the Ascidiae Simplices. 
There are series of forms which bridge over, I believe, every gap which has been 
pointed out as existing between the two groups, and at the present time I find myself 
unable to fix upon a single satisfactory character by which to distinguish the Compound 
from the Simple Ascidians. This is not to be wondered at, however, when we consider 
the great amount of individual variation, and the difficulty of defining the limits of 
allied species and genera in the Tunicata ; and it even seems perfectly natural when we 
investigate the course of the phylogeny of the group, and find that in all probability 
the Compound Ascidians are polyphyletic, having been derived from several distinct 
groups of ancestral Simple Ascidians (see further on in this Eeport under Summary). 
They are thus a semi-artificial assemblage, consisting of those fixed ^ Ascidians which have 
retained or acquired the power of reproducing by gemmation, so as to form colonies, and 
in which the Ascidiozooids have become so intimately united that their tests form a 
common colonial mass. 
Savigny in 1815^ first .rescued the Compound Ascidians from the Alcyonaria with 
which they had previously been associated, and demonstrated their affinity with the other 
Tunicata. In the Tableau Systdmatique, Savigny gives no formal statement of the 
characters by which he distinguished the Simple from the Compound Ascidians, but it 
is evident from some passages in his 3® Mdmoire that he relied chiefly, if not entirely, 
for their separation upon the arrangement of the Ascidiozooids of the ComjDound forms 
around a central cloaca, a character which he declared was visible even in. the young 
emljryo. In this latter point he was mistaken, and it seems rather singular that he 
should have laid such stress upon the union of the atrial apertures when we find that he 
1 Coilocormus huxleyi seems to be unattached, but is probably not free-swimming. 
2 Observations sur les Alcyons g41atineux six tentacles simples. 
