96 
THE VOYAGE OF H.M.S. CHALLENGER. 
In a transverse section of the head three zones may be distinguished (PL X. fig. 5). 
The external one is formed by the transj^arent outer part of the investing mass in which 
are imbedded the light grey thoracic regions of the Ascidiozooids. Tho next or middle 
zone is composed of the transversely cut abdominal regions of a couple of rows of 
Ascidiozooids with very little matrix around them. The third or central zone has matrix 
alone (PI. X. fig. 5), it is rather opaque, and its texture is spongy on account of the 
vascular appendages which traverse it. 
In a vertical section (PI. X. fig. 7) the same three zones are recognised. The 
inclined position of the Ascidiozooids which causes the overlapping is well seen, and it is 
noticeable that the abdomen is usually more nearly vertically placed than the thorax, 
thus accountino- for the fact that more cut ends of the former than of the latter are seen 
O 
in a transverse section (PI. X. fig. 5). In a vertical section (PI. X. fig. 6) the diminution 
in the size of the Ascidiozooids as they approach the base is very evident, but nothing 
resembling buds is visible in the stalk below. At the very base of the head, however, 
and seen in a vertical section as a sort of incomplete cap or funnel-shaped collar sur- 
rounding the top of the stalk where it becomes continuous with the central part of the 
head, is a narrow zone of buds or very young Ascidiozooids lying internal and posterior 
to the lowest row of adults (PI. X. fig. 7), and evidently destined shortly to reach the 
periphery and appear as young but fully developed Ascidiozooids. 
Histology. 
Investing Mass or Test . — The matrix of the head, it has been seen, presents itself 
apparently under tvm different forms ; in the outer thoracic zone it is gelatinous, but solid, 
homogeneous and transparent, while in the centre of the colony it seems firmer, but is 
spongy and irregular, being sometimes reduced to a reticulum, and is more opaque, having 
usually a bluish hyaline appearance. These two parts are, however, identical in structure, 
the difference in their appearance being due to the presence of the vascular appendages 
in the inner resfion of the investino; mass. 
Comparatively little structure is visible in this test. No bladder cells are present in 
any part of the colony, and the other cell elements are comparatively few, the great 
mass of the tissue being homogeneous apparently structureless matrix. The usual 
fusiform and stellate, and many smaller, nearly circular, test cells are present (PI. XL 
fig. 1, t.c.), while here and there another form occurs which may be considered inter- 
mediate between the branched test cell and a bladder cell. These are larger than the 
stellate cells, and are more or less globular or ovate in form ; they have a delicate outline 
from which a few faint hair-like processes radiate outwards like the branches of the 
stellate cells ; the protoplasm does not fill the cell, and it is generally accumulated 
