LARVAL FORMS. 365 



of larvae at all events is adaptive ; and it is probable that organs 

 of locomotion are in many cases specially developed, and not 

 ancestral. 



Various spinous processes on the larvae of Crustacea and 

 Teleostei are also examples of secondarily acquired protective 

 organs. 



These general considerations are sufficient to form a basis for 

 the discussion of the characters of the known types of larvae. 



The following table contains a list of the more important of 

 such larval forms : 



DICYEMID/E. The Infusoriform larva (vol. n. fig. 62). 



PORIFERA. (a) The Amphiblastula larva (fig. 115), with one-half of the body 

 ciliated, and the other half without cilia; (/>) an oval uniformly ciliated larva, which 

 may be either solid or have the form of a vesicle. 



CCELENTERATA. The planiila (fig. 216). 



TURBELLARIA. (a) The eight-lobed larva of Miiller (fig. 222); (b) the larvae of 

 Gb'tte and Metschnikoff, with some Pilidium characters. 



NEMERTEA. The Pilidium (fig. 221). 



TREMATODA. The Cercaria. 



ROTIFERA. The Trochosphere-like larvae of Brachionus (fig. 217) and Laci- 

 nularia. 



MOLLUSCA. The Trochosphere larva (fig. 218), and the subsequent Veliger larva 

 (fig. 219). 



BRACHIOPODA. The three-lobed larva, with a postoral ring of cilia (fig. 220). 



POLYZOA. A larval form with a single ciliated ring surrounding the mouth, and 

 an aboral ciliated ring or disc (fig. 228). 



CH^TOPODA. Various larval forms with many characters like those of the 

 molluscan Trochosphere, frequently with distinct transverse bands of cilia. They are 

 classified as Atrochce, Mesotrochae, Telotrochae (fig. 225 A and fig. 226), Polytrochae, 

 and Monotrochae (fig. 225 B). 



GEPHYREA NUDA. Larval forms like those of preceding groups. A specially 

 characteristic larva is that of Echiurus (fig. 227). 



GEPHYREA TUBICOLA. Actinotrocha (fig. 230), with a postoral ciliated ring of 

 arms. 



MYRIAPODA. A functionally hexapodous larval form is common to all the 

 Chilognatha (vol. n. fig. 174). 



INSECTA. Various secondary larval forms. 



CRUSTACEA. The Nauplius (vol. 11. fig. 208) and the Zoaea (vol. II. fig. 210). 



ECHINODERMATA. The Auricularia (fig. 223 A), the Bipinnaria (fig. 223 B), and 

 the Pluteus (fig. 224), and the transversely-ringed larvae of Crinoidea (vol. II. fig. 268). 

 The three first of which can be reduced to a common type (fig. 231 c). 

 ENTEROPNEUSTA. Tornaria (fig. 229). 



UROCHORDA (TUNICATA). The tadpole-like larva (vol. in. fig. 8). 

 GANOIDEI. A larva with a disc with adhesive papilla? in front of the mouth 

 (vol. in. fig. 67). 



ANUROUS AMPHIBIA. The tadpole (vol. in. fig. 80). 



