MR. HUXLEY ON DOLIOLUM AND APPENDICULARIA. 
599 
Mertens, on the other hand, says, ‘‘The relation of this animal with the Pteropoda 
is unmistakeable ; if the Oikopleura possessed two tail-like appendages, every one 
would recognize in them the wings of the Pteropoda and he proceeds to draw, what 
seems to me, a very forced comparison between Oikopleura and Clio. 
I do not think that any one who has read the preceding pages will be at all dis- 
posed to agree with Mertens either. 
87. For my own part, I think there can be no doubt that the animal is one of the 
Tunicata. The whole organization of the creature, its wide respiratory sac, its 
nervous system, its endostyle, all lead to this view. 
In two circumstances, however, it differs widely from all Tunicata hitherto known. 
The first of them is, that there is only one aperture, the respiratory, the anus opening 
on the dorsum ; and secondly, that there is a long caudal appendage. 
As to the first difference, it may be observed, that, in the genus Pelonaia, an un- 
doubted Ascidian, there are indeed two apertures, but there is no separation into re- 
spiratory and cloacal chambers. Suppose that in Pelonaia the cloacal aperture ceased 
to exist, and that the rectum, instead of bending down to the ventral side of the 
animal, continued in its first direction and opened externally, we should have such 
an arrangement as exists in Appendicularia. 
With regard to the second difference, I would remark, that it is just the existence 
of this caudal appendage which makes this form so exceedingly interesting. 
It has been long known that all the Ascidians commence their existence as larvae, 
swimming freely by the aid of a long caudate appendage ; and as in all great natural 
groups some forms are found which typify, in their adult condition, the larval state 
of the higher forms of the group, so does Appendicularia typify, in its adult form, 
the larval state of the Ascidians. 
Appendicularia, then, may be considered to be the lowest form of the Tunicata ; 
connected, on the one hand, with the Salpce, and on the other with Pelonaia, it forms 
another member of the hypothetical group so remarkably and prophetically indicated 
by Mr. MacLeay, and serves to complete the circle of the Tunicata. 
88. Doliolum. — This name was given by Otto* to a free-swimming gelatinous case, 
altogether structureless, of which a single example was found by him in the Gulf of 
Naples. Its nature is altogether unknown, for it is hardly justifiable, in the face of 
Otto’s words, “ Die Rander sind aber vollig glatt ohne alle Spur von Zerreissung, 
nirgend sieht man inwendig Rauhigkeiten wo die Eingeweide angessessen haben 
konnten und die aussere Haut geht ohne Unterbrechung in die innere fiber,” to 
assume with MM. Quovand Gaimard, that it is only aBiphore whose intestines have 
been destroyed by a parasitic Phronima. 
Furthermore, Otto states that the animal moved by a “ worm-like contraction of 
its walls,” which by no means describes the mode of contraction of the Salpce, with 
which animals he was perfectly acquainted, and with a mutilated specimen of which, 
* Nova Acta Acad. Curiosorum, t. xi. pars secunda, pp. 313 and 314. 
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