EEPOET ON THE THNICATA. 
o 
resemlDlance in shape and general appearance to the Challenger specimens, but differs 
from them in several details. While comparing the two it will be convenient to regard the 
colony as composed of two parts : the head ” or the terminal enlarged part in which the 
Ascidiozooids are imbedded, and the “ stalk ” or elongated peduncle, which bears the head 
at its upper extremity. The lower end of the stalk is attached to some foreign body. 
The general shape of the head and its size relatively to the stalk are the same in the 
figure and in the Challenger specimens, the only point in which they are not absolutely 
identical being that in the “ Astrolabe ” specimen the head evidently did not contract 
cpite so much at its lower end as is the case in those from the Challenger ; this, how- 
ever, is unimportant, as in some specimens it is easy to distort the shape of tlie head by 
allowing it to lie in an abnormal position. 
The colour of the head in the figure is bluish-grey, while that of the Challenger 
specimens is more of a yellowish-grey. 
In the figure the Ascidiozooids are arranged in vertical lines which run distinctly in 
pairs ; further, the Ascidiozooids in a pair of lines are placed alternately so that each has, 
as its neighbours above and below, Ascidiozooids belons;ing, not to its own vertical line, 
but to the adjacent one. The result of this arrangement is that besides vertical lines the 
Ascidiozooids form spirals more or less perfect, especially in the middle part of the head. 
In the Challenger specimens this arrangement, though probably present, is not 
nearly so well marked (see PI. Y. figs. 1, 2, 3). The pairing off of the lines can be made 
out in some heads or in parts of them, but in many places is not at all distinct, and 
in all cases the Ascidiozooids seem to be closer together. Much the same may be said as 
to the alternate position of the Ascidiozooids in the lines ; in some places it is evident 
(PI. Y. fig. 3), in others it is not very distinctly seen (PI. Y. 6g. 2). Still the spiral 
lines are visible here and there. On the whole, the arrangement of the Ascidiozooids in 
the Challenger specimens seems to be more irregular than in the figure of the “ Astrolabe ” 
specimens, though following the same j>lan. 
In shape the stalks are identical in both forms. In colour, however, they differ. 
In the figure the upper third is light indigo-blue, of a darker shade than the head, 
and the lower two-thirds are yellow. In the Challenger specimens the colour varies from 
pale yellow to light reddish-brown, but in none is any part blue ; in some, however, at 
the point of junction with the head, there is a collar of a darker shade of yellow or brown. 
In the other figure (loc. cit., pi. xcii. fig. 19) three apertures are represented and 
seem to be intended for a l^ranchial and two atrials. Both kinds are distinctly six- 
lobed. In the Challenger specimens it is exceedingly difficult to make out the condition 
of the apertures, on account of their contracted state ; they are, however, six-lobed, but 
not nearly so distinctly so as in the figure. Turning now to the short description in the 
text, it is found to give less information than the figures did. Quoy and Gaimard’s 
specific diagnosis is as follows : — “ Aplidium, ovatum, griseo-violaceum, longe pediculatum ; 
