LARVAL FORMS. 301 



prolonged, since, ceteris paribus, the more quickly maturity is reached 

 the better it is for the species, 



A very superficial examination of marine larvae shews that there 

 are certain peculiarities common to most of them, and it is important 

 to determine how far such peculiarities are to be regarded as adaptive. 

 Almost all marine larvae are provided with well-developed organs of 

 locomotion, and transparent bodies. These two features are precisely 

 those which it is most essential for such larvae to have. Organs of 

 locomotion are important, in order that larvae may be scattered as 

 widely as possible, and so disseminate the species ; and transparency 

 is very important in rendering larvae invisible, and so less liable to 

 be preyed upon by their numerous enemies \ 



These considerations, coupled with the fact that almost all 

 free-swimming animals, which have not other special means of 

 protection, are transparent, seem to shew that the transparency 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^. — The Infusoriform larva (vol. i. fig. 62). 



PoRiFERA. — (a) The Amphiblastula larva (fig. 215), with one-half of the body 

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

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



CcELENTERATA. — The planula (fig. 216). 



TuRBELLARiA — (o) The eight-lobed larva of Miiller (fig. 222) ; (6) the larvae of Gotte 

 and Metschnikoff, with some Pilidium characters. 



Nemertea. — The Pilidium (fig. 221). 



Tkematoda. — The Cercaria. 



RoTiFERA. — The Trochosphere-like larvae of Brachionus (fig. 217) and Lacinularia. 



Molltjsca. — 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). 



PoLTzoA. — 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 moUuscan 

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

 as Atrochae, Mesotrochie, TelotrochsB (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 Tobicola. -Actinotrocha (fig. 230), with a postoral ciliated ring of 

 arms. 



Myriapoda. — A functionally hexapodous larval form is common to all the Chilognatha 

 (vol. I. fig. 174). 



Insecta. — Various secondary larval forms. 



Crustacea.— The Nauplius (vol. i. fig. 208) and the Zosea (vol. i. fig. 210). 



1 The phosphorescence of many larvaa is very peculiar. I should have anticipated 

 that phosphorescence would have rendered them much more liable to be captured by 

 the forms which feed upon them ; and it is difficult to see of what advantage it can be 

 to them. 



