DEVELOPMENT. 
211 
great groups of the animal kingdom as to demand notice. 
Most Worms leave the egg as a larva, called the trocho- 
sphere (Fig. 175), an oval larva, having mouth 
and anus, and a circle of cilia anterior to the 
mouth. This larval stage is common to Worms 
with the most diverse adult forms and habits. 
It is also found in all the great groups of Mol- ^oJph^of 
lusks. Clams, Snails, and Cuttle-fish all have worm(Phyi- 
. iodoce): a, 
the stage represented in their history. 1 he Mol- circle of cilia, 
lusks usually pass through a later stage called the veliger 
(Fig. 176), in which 
a circle of cilia ho¬ 
mologous to that of 
the trochosphere is 
borne by a lobed 
expansion on the 
head, called the ve¬ 
lum , or sail. The 
Crustacea, which 
exhibit so great a 
range of form in the adult state, all pass through a stage 
in which they are substantially alike. Forms as different 
in appearance as Barnacles, Entomostracans, and Prawns 
hatch out as Nauplii , little oval animals, with a straight 
intestine, three pairs of legs, and a simple eye (Fig. 177). 
See Figs. 253, 254, 255, 256. Fig. 256 represents the 
Lobster, which does not hatch as a Nauplius, but is not 
very unlike the Prawn. These larval forms are of great 
interest, because they disclose the relationships of the 
adult forms, as the gastrula stage hints at the common 
relationships of all animals above Protozoa. 
2. Alternate Generation . 
Sometimes a metamorphosis extending over several 
generations is required to evolve the perfect animal; “ in 
Fig. 176 _Larval Gasteropoda: A, B, Trochus; C, Ter- 
gipes ; a,, trochosphere ; v, velum ; B, veliger ; d, 
mouth; /, foot; s, shell; C, veliger; d, foot; c, tenta¬ 
cle ; b, ear. 
