60 
THE VOYAGE OF H.M.S. CHALLENGER. 
II. ARE THE PTEROPODA CEPHALOPODA ? 
In the organisation of the Pteropoda certain points may be seen which indicate a 
resemblance to the Cephalopoda. 
1. The ventral position of the pallial cavity of certain Thecosomata (Cavoliniidse and 
Cymbuliidse). 
2. The “ ventral ” flexure of the alimentary canal in the same groups. 
3. The presence of acetabuliferous appendages in some Gymnosomata (Pneumono- 
dermatidse). 
It is impossible, however, to show, as Hyatt 1 maintains, that “ the general aspect, the 
arrangement and position of the oral region, and the disposition of the internal organisa- 
tion are similar in both ” (Pteropods and Cephalopods). This is, indeed, a heresy, and 
one must never have dissected a Pteropod to be able to make such an assertion. 
It has been said that there are three points in which resemblance may be traced 
between the Cephalopoda and Pteropoda. We shall soon see how much foundation these 
resemblances have ; but in the meantime it may be remarked that in not a single point 
of their organisation can a true resemblance be found which would justify the assertions 
which have been made regarding the afiinities of the two groups, nor even explain the 
position which has been assigned to the Pteropods in the neighbourhood of the 
Cephalopods. 
A. The digestive tract and its appendages may be first examined : — 
a. Retractile Proboscis. — This organ, so well developed in the Gymnosomata, 
does not exist in the Cephalopoda. 
b. Radula . — Woodward, 2 speaking of the Cephalopoda, remarks — “ The odonto- 
phore somewhat resembles that of the Pteropods ” — a statement which, 
though quite incorrect, has been copied into other text-books of 
Conchology. 
As a matter of fact this radula of the Cephalopoda is characterised 
by its uniformity ; it always has the formula 3-1-3, or more accurately 
1-2-1-2-1, that is to say, there is a central tooth, and, on either 
side, two lateral teeth and a marginal tooth ; this last differs from the 
two lateral teeth by its general form and by its narrower basilar piece. 
The only exceptions known to this formula, 3-1-3, are Gonatus 
(belonging to the fa mil y Onychii) and Nautilus. The former has no 
1 On the parallelism between the individual and orders in Tetrabranchiate Cephalopoda, Mem Boston Soc. Nat. 
Hist., vol. i. pt. ii. p. 208. 
2 A Manual of the Mollusca, p. 448, 1856. 
