76 Part second. 



generation. In their early youth, these animals are very small, active 

 and free-swimming, with a pear-shaped body and three pairs of swimming- 

 legs. This larval stage is common to all the lower forms of 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 com- 

 pletely 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. 53). 



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 Nr. 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. 



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 all Crustacea. Besides this point they diifer from the above 

 mentioned groups in not being jointed. Very many Mollusks are pro- 

 vided 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 1 arms or feet, arranged 

 in a ring round their mouth, and this has given rise to their scien- 

 tific name. Most striking of these in the Aquarium is the eight-armed 

 Devil-fish (ital. Polpo, the Polypus of the Greek) Octopus vulgaris 

 (Fig. 73 and 74), which is very common on the rocky coasts of the 

 Mediterranean. Its body has the form of a round bag, half of it is hol- 

 low 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 I 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 



