174 ANIMAL METAMORPHOSIS. 



Fig. 28. — Head of male fly : a, root of antenna ; 6, bulb of same ; 

 m, lobate mouth ; j), palp ; o, ocellar prominence. 



,, 20. — Antenna, male. 



,, 30. — Ditto, female. 



A scale consisting of one or more divisions is placed near each 

 figure. Each division represents 10th, 100th, or 400th of an inch, as 

 per table beloAv : — 



10th of an inch, Figs. 1, 7, 10, and 11. 



100th „ Figs. 2, 3, 4, 5, 8, 12, 13, 14, 15, 22, 26, 27, 28, 



29, and 30. 

 400th „ Figs. 6, 9, 10, 17, 18, 19, 20, 21, 23, 24, and 25. 



Hniinal flDetaniorpboeit?. 



By J. B. Jeaffreson, M.R.C.S., Etc. 



Part 2. 



Plate XVII. 



THE majority of the Mollusca undergo a very definite series 

 of changes. Most of them commence life as a small, 

 ciliated, worm-like body, having at its head an expanded 

 lobe, richly clothed with cilia, resembling the trochal disc of 

 a rotifer, by means of which it swims about actively. This 

 process is lost in the adult, and the animal is only able to move 

 about with a slow, creeping mode of progression. The larvae of 

 all bivalves possess eyes, but they are generally lost in the process 

 of development, and when such organs appear in the adult they 

 are of secondary formation. In their embryonic stages all 

 molluscs possess a shell, within which tiie young can entirely 

 retract itself, but in the adult forms of the Nudibranchiata, or 

 Sea-slugs, it is entirely lost or only quite rudimentary. Young 

 oysters, or mussels, are hatched in the gill-chambers of the 

 mother, and swim about freely by means of a ciliated disc at the 

 anterior end of the body. The embryo oyster (Fig. 15) is 



