76 Part second. 



pletely hiding- tlie animal, and only allowing the delicate legs to protrude 

 from a slit-like aperture. These delicate jointed legs can he 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*i» century, in all probability arose from the desire of the priest- 

 hood of that time, to increase the small range of a Lenten l)ill of fare, 

 by deriving- the g-eese 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 Crxistacea. Besides this point they differ from the above 

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

 vided with a shell and have a head verj^ distinctly mairked off from the 

 body and bearing ej^es 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 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; 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 aiiimal 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 those of the membranous flap. The flap allows 

 the water to enter the « mantle-cavity », or holloAv part of the bag, in 

 which hang the gills ; the water which has been used for respiration is 

 then forced out through the « funnel », the flap being kept closed. This 

 alternating- movement serves also as means of progression, the water 

 which is being forced out, propelling- the animal with its hind-end 



