64 
THE VOYAGE OF H.M.S. CHALLENGER. 
a little above in the Limacinidse. Even if the flexure appear neural in the straight 
Thecosomes (the figure of Creseis given by Gegenbaur 1 is inaccurate in this particular), 
it is always in reality lateral, since the anus opens to the left ; and we have seen that 
this difference from the Limacinidse has been caused by the process of rotation already 
explained, and that among the Thecosomata the primitive form is the lateral flexure 
found in the Limacinidse, as also in the Gymnosomata, and differing in both from 
the true neural median flexure of the Cephalopoda. 
In the same w T ay, as regards the pallial cavity of the Thecosomata, it has been shown 
that the primitive form is the dorsal cavity of the coiled Thecosomata, and that the ventral 
position of the pallial cavity in the straight forms is due to a process quite different from 
that which has brought about the analogous situation in the Cephalopoda, and hence that 
the two arrangements are not at all comparable. 
Consequently there is no proof to be found here of any connection between the 
Cephalopods and the Pteropods. 
3. The majority of authors have traced a homology between the buccal appendages of 
the Gymnosomata and the arms of the Cephalopoda. I may specially mention R. 
Leuckart, 2 Loven, 3 von Jhering, 4 Gegenbaur, 5 Grenacher, 6 Brooks, 7 Ray Lankester, 8 and 
Grobben. 9 Huxley alone, 10 even when declaring himself in favour of this interpretation, 
has maintained a certain reservation regarding the innervation of the appendages of the 
Gymnosomata. 
If, however, these authors agree as to the homology of these t-wo sets of organs, they 
differ entirely regarding their morphological value. 
Huxley 11 and Ray Lankester 12 consider them to belong to the foot, whilst, on the other 
hand, von Jhering 13 and Grobben, 14 &c., regard them as cephalic organs. 
Now, I have shown from their innervation that the appendages of the Gymnosomata 
are cephalic in their nature. 
What, then, is the morphological value of the arms of the Cephalopoda ? This 
question, which has been so often discussed, is of great importance. Indeed, it is upon 
the pretended homology between the appendages of the Gymnosomata and the arms of 
1 Untersuchungen uber Pteropoden und Heteropoden, pi. ii. fig. 1, g. 
2 Ueber die Morphologie und die Verwandtschaftsverhaltnisse der wirbellosen Thiere. 
3 Bidrag til Kannedoin om utveckling at' Mollusca Acephala Lamellibranchiata, K. Svensk. Vetensk. Akad. Handl. f 
1848. 4 Vergleichende anatomie des Nervensystemes und Phylogenie der Mollusken. 
6 Grundriss der vergleichenden Anatomie. 
6 Zur Entwickelungsgeschichte der Cephalopoden, Zeitschr.f. wiss. Zool., Bd. xxiv. 
7 Development of the Squid, Loligo Pealii, Anniv. Mem. Boston Soc. Nat. Hist., 1880. 
8 Mollusca, Encyclopaedia Britannica, 9th ed. vol. xvi. 
8 Zur Kenntniss der Morphologie und der Verwandtschaftsverhaltnisse der Cephalopoden, Arb. Zool. Inst. 
Wien, t. vii. 10 On the Morphology of the Cephalous Mollusca, PM. Trans., 1853, p. 40. 
11 Loc. cit. pi. v. fig. 5. 12 Mollusca, Encyclopaedia Britannica, 9th ed. vol. xvi. p. 664. 
13 Vergleichende Anatomie des Nervensystemes und Phylogenie der Mollusken, p. 269. 
14 Zur Kenntniss der Morphologie und der Verwandtschaftsverhaltnisse der Cephalopoden, Arb. Zool. Inst. Wien, 
vii. p. 71. 
