LARVAL FORMS : THEIR NATURE^ ORIGIN AND AFFINITIES. 385 
widely as possible, and so disseminate the species ; and trans- 
parency 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 show that, at 
all events, the transparency of larvae 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 : 
DiCYEMiDiE. — The Infusoriform larva. 
PoEiEERA.— («) The Amphiblastula larva (fig. 1), with one half of the 
body ciliated, and ‘the other half without cilia; (b) the oval uniformly 
ciliated larva, which may be either solid or have the form of a vesicle. 
CcELENTEEATA. — The plauula (fig. 2). 
Turbellaria. — {a) The eight-lobed larva of Muller (fig. 9); {b) the 
larvae of Gotte and Metsclmikoff, with some Pilidium characters. 
Nemertea. — The Pilidium (fig. 8). 
Trematoda. — The Cercaria. 
lloTiEERA.— The Trochosphere-like larvae of Brachionus (fig. 3) and 
Lacinularia. 
Mollusca. — The Trochosphere larva (fig. 4), and the subsequent Veliger 
larva (fig. 5). 
Brachiopoda. — The three-lobed larva, with a postoral ring of cilia (fig. G). 
PoLYZOi. — A larval form with a single ciliated ring surrounding tlie 
mouth, and an aboral ciliated ring or disc (fig. 15). 
CH.ETOPODA. — Various larval forms with many characters like those of 
Molluscan Trochosphere frequently with distinct transverse bands of cilia. 
They are classified as Atrochse, Mesotrochse, Telotrochse (fig. 12 a and fig. 
13), Polytrochee, and Monotrochse (fig. 12 b). 
Gepiiyrea Nuda. — Larval forms like those of preceding groups. A 
specially characteristic larva is that of Echiurus (fig. 14). 
Gephyrea Tubicola. — Actiuotrocha (fig. 17) ; with a postoral ciliated 
ring of arms. 
Myriapoda. — A functionally hexapodous larval form is common to all 
the Chilognatha. 
Insect A. — Various secondary larval forms. 
Crustacea.— The Nauplius and the Zoeea. 
Eouindermata. — The Auricularia (fig. 10 a), the Bipinnaria (fig. 10 b), 
and the Pluteus (fig. 11), and the transversely-ringed larvae of Criuoidca 
^ The phosphorescence of many larvm 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 diflicult 
to see of what advantage it can be to them. 
