48 
Tanks 10, 15 and 22. 
Crustacea and is termed the "Nauplius” stage. But after several moults 
this larva fixes itself by its head to some convenient object, and now the 
skin begins to secrete the calcareous covering, which consists of several 
plates completely hiding the animal, and only allowing the delicate legs 
to protrude from a slit-like aperture. These delicate jointed legs can be 
seen waving perpetually in both Balanus and Lepas, as they so serve to 
create a current bringing food to the mouth (see p. 14). 
Balanus forms a belt all round the rocks just at the surface of the 
water, and these barnacles are so securely attached that they cannot be 
washed off by the waves. At low-tide they bear the greatest heat of the 
sun , lasting out till high-tide with the least drop of water, which they 
retain in their tightly closed shell. Lepas (when present, in tank No. 10 
or 22) prefers to attach itself to floating objects; it is found in large 
numbers on ships, driftwood, etc. Its name of Goose-barnacle is due to 
the fable according to which they are the young of the goose called, after 
them, the Barnacle-goose. This myth, which is traced back to the end 
of the 12 th century, in all probability arose from the desire of the priest¬ 
hood of that time, to increase the small range of a Lenten bill of fare, 
by deriving the geese from marine animals. 
MOLLUSCA. 
(Tanks No. 3, 15, 19, 20, 22 and 24.) 
The term Mollusks, i. e. soft-bodied animals, is applied to Snails, 
Slugs, Poulps, Bivalves and their allies, because they have no skeleton 
which enters into the mechanism of their movements; neither an internal 
one like that of the Vertebrates, nor an external one like that of many 
Worms and Crustacea. Besides this point they differ from the above 
mentioned groups in not being jointed. Most Mollusks are provided with 
a shell and have a head very distinctly marked off from the body and 
bearing eyes and tentacles. 
We will begin with the highest group of the Mollusks, that of the 
Cephalopoda or Poulps. Their head bears 8 or 10 arms or feet, arranged 
in a ring round their mouth, and this has given rise to their scientific 
name. Most striking of these in the Aquarium is the eight-armed Devil-fish 
(ital. Polpo, the Polypus of the Greek), Octopus vulgaris (Fig. 78 and 79, 
Tank 15), which is very common on the rocky coasts of the Mediterranean. 
Its body has the form of a round bag, half of it is hollow and performs 
regular respiratory contractions; at one end is a toad-like head provided 
with two large eyes, and with eight arms united at their base by a web; 
each arm bears two rows of suckers. Hidden in the middle of the arms 
is the mouth furnished with a pair of jaws, having the form of a parrot’s 
beak. As the animal breathes, there may be noticed a membranous 
flap which alternately opens and shuts, and laterally a projecting 
tube ("funnel”) which also opens and shuts, its movements alternating with 
that of the membranous flap. The flap allows the water to enter the 
"mantle-cavity”, or hollow part of the bag, in which hang the gills; the 
water which has been used for respiration is then forced out through the 
