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- 

 lusks. Clams, Snails, and Cuttle-fish all have 

 the stage represented in their history. The Mol- 

 lusks usually pass through a later stage called the veliger 



(Fig. 176), in which 







H* 



" m 



a circle of cilia ho- 

 mologous to that of 

 -^ the trochosphere is 

 borne by a lobed 

 expansion on the 

 head, called the ve- 

 or sail. The 



Fio. 176 Larval Gu.tempoJ.a: A, /j, Trochus; C, Ter- 

 gipes ; a, trochosphere ; v, velum ; B, veliger ; d, ~ . . 



mouth ; /, foot; a, shell; C, veliger; d, foot; c, teiita- Crustacea, WlllCll 



cle;& ' ear> 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, Entornostracans, 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 



