404 ANIMAL LIFE-HISTORIES 



noise, or a very gentle tap upon the vessel containing the pupae, 

 causes those at the surface to dart downwards, but as slight sounds 

 of various kinds produce no effect upon them, I conclude that the 

 tremor of the surface of the water, and not the sound itself, was 

 recognized by them." The last stage in the life-history is reached 

 when the adult gnat makes its way out of the floating pupa, which 

 splits along its back, and the wings are very quickly ready for 

 use: otherwise there would be considerable danger of drowning. 



LIFE-HISTORIES, &c., OF MOLLUSCS (MOLLUSCA) 



We have here to successively consider members of the follow- 

 ing subdivisions: — (i) Primitive Molluscs (Amphineura), (2) Bi- 

 valves (Lamellibranchia), (3) Tusk-Shells (Scaphopoda), (4) Snails 

 and Slugs (Gastropoda), and (5) Head-Footed Molluscs (Cephalo- 

 poda). 



Primitive Molluscs (Amphineura). — The only members of 

 this group about which we possess detailed knowledge as regards 

 life-history are the curious flattened Mail-Shells (species of Chiton, 

 &c.), most of which live in shallow water, shel- 

 tered under stones or in narrow rock-crevices, 

 to which their shape is no doubt an adaptation. 

 The back is protected by eight overlapping 

 shelly plates, and outside these the body-wall 

 is drawn out into a narrow flap (mantle) under 

 which the gills are lodged (see vol. ii, p. 392). 

 Fig. 925. - Trochosphere The eggs arc covered by firm shells, with 



Larva of a Rlall-Sheli (C/izVtJ?;i, . • . -. . . / 



enlarged; m, mouth. spmy projections, and m some species (e.g. 



Chiton Po/ii) they remain under the shelter 

 of the mantle till the time of hatching. The development is 

 indirect, and the just-hatched young is a ciliated Trochosphere 

 (fig. 925), which resembles in many ways the larva of the same 

 name characteristic of marine Bristle- Worms (see p. 359). Cuttle- 

 fishes and the like excepted, most molluscs begin independent 

 existence in the form of trochospheres, or else as larvae which 

 may be regarded as derived from these. This fact has been 

 adduced by some zoologists in support of the view that the remote 

 ancestors of Molluscs resembled marine Bristle- Worms in essential 

 respects. If this "Annelid Theory" of molluscan descent be 

 accepted, molluscs are profoundly different from their ancestors, 



