8 
THE VOYAGE OF H.M.S. CHALLENGER. 
already said that in Limacina helicina and Limacina antarctica the operculum is caducous 
in fully-grown specimens, a fact which explains how it is that these species have often 
been regarded as lacking this organ. 
The Mantle is open dorsally, and united to the body behind the foot (on the ventral 
surface of a spread out Limacina). Its margin is simple ; it presents, on the right side, 
a little ventrally, a rather narrow lobe, terminating in a point, and called the “ balancer,” 
the considerable development of which perhaps enables it to play the part of a counter- 
poise during swimming, the coiled Pteropods not being symmetrical like the straight 
forms. Possibly this lobe is also sensory, as its whole surface is ciliated. 
The dorsal portion of the mantle which covers the pallial cavity presents a rather thick 
glandular area, corresponding to the “ shield ” of the Cavoliniidse. The structure of this 
organ is already known and is practically the same in Limacina as in the Cavoliniidse ; 
but in the present case it is asymmetrical (PI. I. fig. 5, a) and uniform in structure 
throughout its extent. 
The Digestive Tract. — The mouth opens in the natatory plane formed by the two fins 
enveloping the cephalic region ; it is situated towards the dorsal border of this plane and 
bounded by two lips, united dorsally and separating towards the other side. 
The mouth is succeeded by a buccal mass, the cavity of which encloses two lateral 
jaws, such as have already been described by Sars. 1 The number of folds presented 
by these jaws varies in different species. The disposition of the radula agrees with that of 
all the odontophorous Mollusca, but the ribbon is very short, the number of transverse 
rows being but small. The number of longitudinal series is three, as in all the Theco- 
somata. 
On either side of the radula opens a salivary gland. These organs have escaped the 
attention of the different naturalists who have studied Limacina (van Beneden, 2 Souleyet 3 ). 
According to Gegenbaur 4 they are wanting in all Thecosomata, nevertheless all these are 
provided with them. In Limacina these glands are small, short, oval, and without a 
differentiated duct. 
The oesophagus, rather long and longitudinally plicated within, leads into an enlarge- 
ment of the digestive tube called the stomach, which here, as in all Thecosomata, is in 
reality a masticatory gizzard. Its walls have about the centre a large muscular transverse 
band, which actuates a number of horny plates situated within it. 
These masticatory plates are four in number and are placed symmetrically (two 
ventral and two dorsal) ; a fifth has not been observed by the anatomists, although the 
embryologists have recorded its existence (Krohn, 5 Fol 6 ). The four symmetrical plates 
1 Mollusca regionis arctic® Norvegiee, pi. xvi. fig. 17. 2 Memoire sur la Limacina arctica, loc. cit. 
3 Voyage de la Bonite, Zoologie, t. ii. p. 210. 
4 Untersuchungen fiber Pteropoden und Heteropoden, p. 10. 
6 Beitrage zur Entwickelungsgeschichte der Pteropoden und Heteropoden, p. 42. 
0 Sur le developpement des Pteropodes, Arch, de Zool. Expdr., ser. 1, t. iv. p. 162. 
