382 
THE VOYAGE OF H.M.S. CHALLENGER. 
marked peduncle, upon which the remainder of the colony (the “head”), containing the 
Ascidiozooids, is borne. Gemmation is effected by means of the vascular prolongations 
from the Ascidiozooids which penetrate the peduncle, and the history of the colony in 
Colella appears to be as follows : — The first Ascidiozooid produced by the fixing of the 
tailed larva forms the young test which grows rapidly at the side next the point of attach- 
ment, thus raising the Ascidiozooid upwards on a short stalk, A vascular prolongation 
from the posterior end of the body of the Ascidiozooid now grows downwards ‘ into the 
gradually increasing peduncle, and after a time forms one or more young buds, which 
become cut off from the vascular prolongation and lie in the test of the peduncle. These 
buds are now carried upwards by the constant growth of the peduncle at its base, so that 
eventually, when they are fully developed, they have reached what is now the upper part 
of the peduncle, and take their places in the “ head ” next to the Ascidiozooid from which 
they were originally produced. Meanwhile the vascular prolongation from that Ascidio- 
zooid has been growing downwards still further through the peduncle and producing 
new buds. The young Ascidiozooids formed from the first buds in their turn produce 
vascular prolongations, and then buds in the peduncle. As the Ascidiozooids in the upper 
part of the colony or “head” grow older they produce true reproductive organs, and 
embryos are formed. These undergo their development in the incubatory pouch, a large 
diverticulum from the dorsal edge of the peribranchial cavity. The oldest Ascidiozooids 
and embryos in the colony finally reach the upper end, die, and decay, the fully developed 
tailed larvae in the incubatory pouches being set free to swim away and found new 
colonies. From this account it is obvious that the test grows at the lower end of the 
colony, and is constantly dying and wasting away at the upper end, while each Ascidio- 
zooid in the course of its life traverses the whole length of the colony (see p, 93). 
On account of the large size of the Ascidiozooids in two of the species of Colella, 
of which there were duplicates in the collection, these forms were chosen for a detailed 
examination of the anatomy and histology (see pp. 74 and 94), 
The pigmentation of some of the species of Colella is very striking. In Colella thomsoni 
the body of the Ascidiozooid is coloured of a deep indigo-blue, while in Colella murrayi 
the test contains great masses of opaque white pigment cells. The alhed genus Distaplia 
also shows vivid pigmentation. Distaplia vallii is coloured by red, opaque white, and 
dark violet pigment cells. 
Two new species were found belonging to Cystodytes, a genus characterised by the 
possession of discoid calcareous spicules in the test , around the bodies of the Ascidiozooids. 
These spicules seem to be enclosed in membranous capsules, containing nuclei, and this 
fact, taken along with some observations made upon the relations of the spicules to. the 
test in Leptoclinum tonga (see p. 269), seems to point to the origin of these calcareous 
spicules from groups of modified test cells, the remains of which persist around the 
spicule in the form of a delicate membrane. 
