Q4 



Part second. 



other snails, but is fixed to one spot. At first sight its shells are 

 just like the calcareous tubes of Protida (see p. 76). 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 them- 

 selves into their tubes. 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. In the Aquarium the 

 tubes grow towards the strongest light. 



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. 18), 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 on 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 No. 3 may 

 be further observed the two flattened snails Pleurobranchus 

 (fig. 21) and Umbrella (fig. 19). 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. 102). Its body is white and semi-transparent, and carries an 

 exceedingly broad head. On both sides of the back are attached 

 the dehcate transparent gills, and besides these there are large 

 spotted appendages which fall off very easily and were therefore 

 formerly considered as parasites. Tethys is not rare in the Gulf 

 and is scooped up from the surface of the sea in buckets by the 

 fishermen. Like the pelagic animals, however, it only lives for 



