

THE ANIMALS OF THE FRESHWATER AQUARIUM 125 



pairs of legs instead of three, and by having no antennas on the 

 head region. Like an insect it is, however, an air-breather. The 

 method of breathing is not dissimilar to that adopted by many 

 aquatic insects, that is, the under surface of the abdomen iscovered 

 with fine hairs in which bubbles of air become entangled at the 

 surface. As the spider descends in the water therefore it shines 

 like quicksilver, owing to the film of air which it carries with 

 it. This supply is only limited, and therefore it must rise to 

 the surface frequently. To obviate this it has the curious habit 

 of spinning a little cell like an inverted cup, attached by threads 

 to water- weed. As it is formed this cell naturally fills with water, 

 but the spider ascends to the surface and descends repeatedly 

 with a cargo of air bubbles which are discharged into the cup, 

 displacing the water in the same way as the oxygen in the experi- 

 ment, recommended on page 71, displaces the water in the bottle. 

 When the cell is filled with air the spider rests in it, finding for a 

 time enough air to enable it to dispense with frequent ascents. 

 Here also the eggs are laid in a little cocoon, and are watched 

 over by the mother until they hatch. The spider shows relat- 

 ively few adaptations to the aquatic life, and the fact that it 

 may leave the pond and run about on its banks in search of 

 prey suggests that the aquatic habit is of relatively recent 

 origin. 



Of the animals simpler in structure than insects and spiders 

 not many are available for the purposes of the school freshwater 

 aquarium. If desired the ringed worms may be represented 

 by the medicinal leech purchased at the chemist's, or by the 

 horse-leech so commonly found in ponds, but neither is very 

 interesting. The horse-leech, which has not the beautiful bands 

 of colour seen on the upper surface of the body of the medicinal 

 leech, sometimes attacks frogs, but feeds for the most part on 

 worms and larvae, while the medicinal leech is purely a blood- 

 sucker. Both are chiefly interesting because of their adapta- 

 tions to a parasitic or semi-parasitic mode of life, and this is 

 a point on which it is hardly advisable to dwell in the case of 

 junior pupils. Again, though we have some beautiful fresh- 

 water Polyzoa, yet the fact that it is not possible to make anything 

 of the structure without the aid of a microscope excludes them 



