BEVELOPMENT. 



211 



Fig. 17s.— TiM- 

 chosphere of 

 Worm (Phyl- 

 lodoce ) : a, 

 circle of cilia. 



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 tiie 

 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- 

 Insks usually pass tlirough a later stage called the velige¥ 



(Fig. 176), in which 

 Q a circle of cilia ho- 

 mologous to that of 

 the trochosphere is 

 jj borne by a lobed 

 expansion on the 

 I lead, called the ve- 

 The 

 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 Mawplii, 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, wliich does not hatch as a Nauplius, but is not 

 very uTilike tiie Prawn. These larval forms ai'e of great 

 interest, because they disclose the relationships of the 

 adult forms, as the gastrula stage hints at the common 

 relationsliips of all animals above Protozoa. 



2. Alternate Generation. 

 Sometimes a metamorphosis extending over several 

 generations is required to evolve the perfect animal; "in 



Fio. 176 — Lavvnl GiiBteiopoila: ^, £, TidchaBj C, Ter- lum, Or Sail, 



gipes J A, trochosphere : «, velum ; B, veliger ; d, _. 



mouth ; /, foot ; 8, shell ; C, veliger ; d, foot ; c, tenta- O T U S t a C 6 a, 

 cle; &, ear. Magnified. 



