80 * Part second. 



of tkis acid fluid in order to soften the hard calcareous skin of the Echi- 

 noderms which it eats, and perhaps also in self-defence; it is still a puzzle 

 how these strong corrosive acids are produced by the animal arid how 

 they can be stored up. 



An interesting animal is the Worm-shell. VemietUS (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 calca- 

 reous tubes of Protula (see p. 66). But on closer inspection may be re- 

 cognized 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 tubes. To the inner wall of the latter 

 they also fix their eggs ; from these free-swimming larva? are hatched, 

 which afterwards fix themselves to some rock. 



Allied to the above mentioned snails 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 &re carried erect 

 and are something like the ears of a hare. The rnautle 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 protec- 

 tion 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 on vegetable matter and graze in hundreds 

 on the fields of sea-weeds. It is interesting to see them when stones co- 

 vered 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 , cf 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 exceedingly 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 parasites. This 

 animal , like other pelagic ones, is only occasionally found in the Aqua- 

 rium; the fishermen scoop them up from the surface of the sea with 

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



