568 
MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
4. It is proposed in the following pages to consider — 
I. The structure of the species of Salpa examined. 
II. The structure of Pyrosoma. 
III. The homology of structure of Salpa Pyrosoma, and of these with the ordi- 
nary Ascidians. 
IV. The history of our knowledge of the Salpce. 
I. The Structure Salpa. 
5. Before entering upon this question, there is a point of some importance to be de- 
termined, as to the upper and lower surfaces, the anterior and posterior extremities 
of the Salpce. Observation will not decide this apparently simple matter, for as the 
writer has frequently seen, they swim indifferently with either end forward and with 
either side uppermost ; and the determinations of authors are most contradictory. 
Throughout the present paper, that side on which the heart is placed will be 
considered as the dorsal side ; that on which the ganglion and auditory vesicle are 
placed, as the ventral side. That extremity to which, the mouth is turned will again 
be considered as the anterior extremity, the opposite as the posterior. Such a view 
of the case appears to be more harmonious with the determinations of corresponding 
parts in other animals than any other. In all the invertebrata the mouth end is always 
considered as the anterior, the heart side as the dorsal side. 
6. The two so-called species of Salpa examined were the S. democratica of 
Forskahl, spinosa of Otto, and the S. mucronata of Forskahl, pyramidalis of Quoy 
and Gaimard. They are described by Sars as S. spinosa and S. mucronata, or rather 
as S. spinosa, proles solitaria, and S. spinosa, proles gregaria. This however begs the 
question, as to the truth of Chamisso’s theory, and I shall therefore prefer to name 
the two forms I observed simply Salpa A and Salpa B. 
At Cape York, and only there, these two forms were always obtained together. They 
were of about the same size, but so totally distinct in appearance, that, had they be- 
longed to any other genus, they would have been justly regarded as separate species. 
7. Salpa A (Plate XV. fig. 1). — The body is gelatinous, transparent and colourless, 
except the nucleus (?'), which has a deep reddish-brown tint. It has a general square 
prismatic shape, and is abruptly truncated and somewhat convex at each extremity. 
The posterior extremity is provided with eight hornlike processes, which project back- 
wards. Two of these are short and hook -like, placed one before the other in the 
median line at the posterior part of the superior surface. On the upper part of the 
in the Ann. des Sciences for 1846, has completely anticipated the chief results arrived at, a fact of which the 
author was totally unaware until his arrival in England in the end of 1850. 
Still, as M. Krohn gives merely his conclusions without details or figures, his promised memoir not having 
appeared (so far as the writer of the present paper is aware), it is hoped that this anticipation will, by showing 
that perfectly independent observers arrive at the same result, rather tend to increase than to diminish any 
weight that may be attached to the present researches. 
