34 
THE YOYAGE OF H.M.S. CHALLENGEK. 
gelatinous films formed by some species of Botryllus, and great thick lobed solid 
masses as in Sarcobotrylloides, may be found, and they all belong to the family 
Botryllidse. 
The systems vary greatly in shape in the different genera, but they are always 
conspicuous, and usually clearly defined (see Fig. 7, p. 35). The common cloaca of the 
system is usually large and conspicuous, even in spirit specimens. It has a more or 
less well-marked membranous margin and is frequently lobed, a lobe corresponding to 
each Ascidiozooid in the system. 
The Ascidiozooids are distinctly marked on the outside of the colony, and are 
characterised by their short and undivided bodies, showing no distinction between 
branchial and visceral regions such as is found in most other Compound Ascidians. The' 
branchial aperture is described by von Drasche as “toothless” in his definition, but this 
cannot be accepted as an invariable characteristic, as Botrylloides purpureum has the 
branchial aperture distinctly lobed' (see p. 43 and PI. II. fig. 7, hr). The atrial aperture 
is generally provided with a single lobe or projecting languet which joins the membranous 
margin of the common cloaca. 
The common test is soft and gelatinous. It is usually very transparent, and contains 
a great many branched “ vessels ” or ectodermal processes from the Ascidiozooids contain- 
ing blood and ending in swollen knobs or bulbs. The branchial sac is always of very 
large size, larger than in any other fa mi ly of the Ascidise Composite. It extends over 
the entire length and breadth of the body of the Ascidiozooid, the alimentary and 
reproductive viscera being placed alongside it. There are usually about twelve rows of 
stigmata, and at the middle of the series there are from ten to thirty in each row. The 
dorsal lamina is a plain narrow membrane which may be corrugated, but has no 
distinct teeth nor ribs. The tentacles are not large, and never exceed sixteen in 
number. They may be reduced to two. Four larger and four smaller, placed alternately, 
is the characteristic arrangement. 
The alimentary canal lies on the side of the posterior one-third or one-fourth of the 
branchial sac, and is mainly directed transversely. The stomach is large, and is usually 
globular in shape. It is folded longitudinally. On each side of the body, about the 
middle, lies the ovary, more or less surrounded by the spermatic vesicles. Thus two 
hermaphrodite genital glands are present. Gemmation usually takes place from the 
sides of the Ascidiozooids, but in Sarcobotrylloides at least buds may also be formed 
in the dilatations on the ectodermal processes or vessels of the common test (see 
PI. IV. fig. 13). Consecjuently the lateral budding can no longer be regarded, as 
it was l>y von Drasche, as a diagnostic feature of the Botryllidse, although it is 
certainly the characteristic method of reproduction by gemmation in the family. The 
exceptional method found in Sarcobotrylloides will be discussed at length further on 
(see p. 59). 
