80 Part second. 



'/2 per cent free hydrochloric acid. The animal squirts out considerable 

 amounts of this acid fluid in order to soften the hard calcareous skin 

 of the Echinoderms which it eats , and perhaps also in self-defence ; it 

 is still a puzzle how these strong corrosive acids are produced by the 

 animal and how they can be stored up. 



An interesting animal is the Worm-shell, Vermetus (Fig. 125), from 

 the fact that it is not able to move about freely like the other snails, 

 but is fixed to one spot. At first sight its shells are just like the 

 calcareous tubes of Protula (see p. 66). But on closer inspection may 

 be recognized the head of the snail with its two short tentacles, very 

 different from the brightly coloured gills of the worms. These animals 

 feed on the small Crustacea and worms in their neighbourhood, and when 

 alarmed, they withdraw themselves into their tube. To the inner wall 

 of the latter they also fix their eggs; from these free-swimming larvae 

 are hatched, which afterwards fix themselves to some rock. 



Allied to the above mentioned forms is a group of Mollusks which 

 differ in having their gills attached behind their heart instead of in front. 

 They are devoid of a shell or only possess a very small one which is 

 hidden by the mantle, as in the land-slugs. This group includes: 



The Sea-hare Aplysia (Fig. 16), a fairly large dark-brown animal 

 with two pairs of tentacles, the posterior pair of which are carried erect 

 and are something like the ears of a hare. The mantle is continued 

 into two large wing-like flaps. They generally creep about lazily over 

 the rocks; but if they want to swim, they begin to flap their "wings", 

 till they have raised themselves from the ground. Once afloat they swim 

 very well and fairly fast, but not for long. When annoyed they give 

 out a very beautiful violet, and also a white liquid, which serves as 

 a protection like the sepia of the Cuttlefish. The ancients already were 

 aware of the poisonous nature of the white secretion, for their writers 

 record its use for the preparation of poisonous and magic draughts; 

 those who partook of these were supposed to linger on in agonies until 

 the Sea-hare died. The Sea-hares live 011 vegetable matter and graze 

 in hundreds on the fields of sea- weeds. It is interesting to see them 

 when stones covered with sea-weeds are brought into their tank ; they 

 creep forth from every corner to get at the food, and in a few hours the 

 stones are bare. With the help of this food they live a long time in 

 the Aquarium; they grow to considerable size and often lay their eggs 

 in long thread-like masses, of yellow or violet colour, on the walls of 

 the tank. In tank Nr. 3 may be further observed the two flattened 

 snails Pleurobranchus (Fig. 17) and Umbrella (Fig. 18). The shell of 

 the latter is quite flat and the foot relatively very high. 



One of the finest animals belonging to this group is Tethys 

 (Fig. 101). Its body is white and semi-transparent, and carries an exceed- 

 ingly broad head. On both sides of the back are attached the delicate 

 transparent gills, and besides these there are large spotted appendages 

 which fall off very easily and were therefore formerly considered as para- 

 sites. This animal, like other pelagic ones is only occasionally found in 

 the Aquarium; the fishermen scoop them up from the surface of the sea 

 with buckets. They swim by throwing their head first to one side and 



