12 
THE VOYAGE OF H.M.S. CHALLENGER. 
ANATOMY. 
The following account of the anatomy and histology ^ of the Compound Ascidians 
deals chiefly with the points in which the groups of these Tunicata differ from the 
Simple Ascidians and from one another in the arrangement and structure of their 
various organs, and therefore may be regarded as supplementary to the corresponding 
section of the Introduction to Part I. of this Eeport. 
The minute anatomy of certain special forms {e.g., Colella pedunculata, p. 77, and 
Colella thomsoni, p. 95) will be found in their proper places in the systematic part of the 
work ; and the more important points of novelty and interest exhibited by the Challenger 
collection will be discussed in the General Summary at the conclusion of the Eeport. 
Classification. 
The scheme of classiflcation given in Part I. (p. 25) is retained unchanged, except 
that the families Coelocormidse and Polystyelidse have been added to Suborder II., the 
Ascidise Compositse. The same system of nomenclature will still be made use of, and 
the body, when being described, is supposed to be in the position flgured and explained 
on p. 27 of Part. I. 
The Colony. 
As aU Compound Ascidians reproduce by gemmation, specimens are almost always 
found to be colonies consisting of a number of members. These members may be^called 
Fio. 1. — A Colony of a Compound Ascidian [Botryllus). The Ascidiozooids are arranged in eight systems. 
a, a large system formed of fifteen Ascidiozooids : h, a small system formed of seven Ascidiozooids ; c, the test or invest- 
ing mass ; d, a common cloacal aperture ; e, an Ascidiozooid ; /, the terminal knobs of the vessels ; g, a system 
where the common cloacal aperture is fully expanded. 
“Ascidiozooids,”* and the colony as a whole the “ Ascidiarium ” or “ Cormus.” If the 
Ascidiozooids are arranged in definite groups which can be distinguished from one 
* For further details in regard to the structure of the Compound Ascidians I may refer the reader in particular 
to Giard’s Recherches sur les Synascidies, and to Della Valle’s Nuove Contribuzioni alia Storia Naturale delle 
Ascidie Composte ; and also to the account of the structure of Colella pedunculata given further on in this Report (p. 74). 
* This term was first used, I believe, by Huxley. 
