REPOET ON THE TUNICATA. 
13 
another, then the colony may he dmded into “ systems ” or “ coenobii,” each system 
consisting of a group of Ascidiozooids ’^dth the surrounding part of the common test or 
investing mass (see fig. 1, p. 12). 
AYhen there are no systems present the Ascidiozooids may either be scattered equally 
over the colony or be placed irregularly. In the Clavelinidse, the only Simple Ascidians 
which reproduce by gemmation, the members of the colony are never arranged in 
systems. Usually, in the Compound Ascidians, the various Ascidiozooids of a system 
have their atrial apertures opening into a chamber, the common cloaca, which com- 
municates with the exterior, and thus serves as the common excretory orifice of a 
number of Ascidiozooids (see fig. 1, where g shows the atrial ends of the Ascidiozooids 
projecting into the common cloaca; see also fig. 3 on p. 16). 
The Test. 
The ground mass or investing substance of the colony (c in fig. l), in which the 
Ascidiozooids are imbedded, is homologous with the test or outer tunic of the Simple 
Ascidians, and ought to be called by the same name. Looked at merely from an ana- 
tomical and not from an eml)ryological point of view, a Compound Ascidian colony may 
be regarded as a number of Simjfie Ascidians whose tests have completely fused with one 
another to form a common mass. In some cases this mass in place of being the same 
throuo-hout is modified in the immediate neisjhbourhood of the Ascidiozooids, so that 
each member of the colony seems to have a test of its own, and these tests are then 
united by the colonial investing mass. Such an arrangement is, however, rarely met 
with. 
In structure, the test of the Compound Ascidians agrees very closely with that of 
the Ascidise Simphces, consequently it is unnecessary to repeat the general characters 
given in the Introduction to the first part of this Keport.^ In most cases it remains in 
a comparatively unmodified condition, corresponding to that shown by the Clavelinidse and 
the Ascidiid^ amongst Simple Ascidians ; and it never assumes the fibrous condition 
found in most Cynthiidae. As a rule it is soft, gelatinous, and semi-transparent, and 
exhibits no difFerentiation to the eye, except occasionally {e.g., Botryllidae) when 
“ vessels ” are particularly well developed (/in fig. 1 ; see also PI. I. figs. 2 and 5). It may 
be rendered opaque by foreign bodies such as sand-grains adhering to or becoming 
imbedded in its surface (as in the case of many of the Polyclinidae, e.g., Psamma]glicliwn, 
p. 237, and PI. XXXI. fig. 9), or by the formation of pigment or of calcareous spicules in the 
interior. The matrix is homogeneous, or rarely exhibits a very delicate fibrillation under 
a high magnification ; occasionally the outer layer is found to be slightly fibrous in 
structure. The cells in this matrix are mostly of small size, but may be of almost any 
* Vol, vi. part xvii. p. 28 , 1882 . 
