Various spinous processes on the larvæ 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 larvæ.
The following table contains a list of the more important of such larval forms:
Dicyemidæ.—The Infusoriform larva (vol. II. 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 solid or have the form of a vesicle.
Cœlenerata.—The planula ([fig. 216]).
Turbellaria.—(a) The eight-lobed larva of Müller ([fig. 222]); (b) the larva of Götte and Metschnikoff, with some Pilidium characters.
Nemertea.—The Pilidium ([fig. 221]).
Trematoda.—The Cercaria.
Rotifera.—The Trochosphere-like larvæ of Brachionus ([fig. 217]) and Lacinularia.
Mollusca.—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]).
Chætopoda.—Various larval forms with many characters like those of the molluscan Trochosphere, frequently with distinct transverse bands of cilia. They are classified as Atrochæ, Mesotrochæ, Telotrochæ ([fig. 225] A and [fig. 226]), Polytrochæ, and Monotrochæ ([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. II. fig. 174).
Insecta.—Various secondary larval forms.
Crustacea.—The Nauplius (vol. II. fig. 208) and the Zoæa (vol. II. fig. 210).
Echinodermata.—The Auricularia ([fig. 223] A), the Bipinnaria ([fig. 223] B), and the Pluteus ([fig. 224]), and the transversely-ringed larvæ 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. III. [fig. 8]).
Ganoidei.—A larva with a disc with adhesive papillæ in front of the mouth (vol. III. [fig. 67]).
Anurus Amphibia.—The tadpole (vol. III. [fig. 80]).
Fig. 215. Two free stages in the development of Sycandra raphanus. (After Schultze.)
A. Amphiblastula stage.
B. Stage after the ciliated cells have commenced to be invaginated.
c.s. segmentation cavity; ec. granular epiblast cells; en. ciliated hypoblast cells.
Fig. 216. Three larval stages of Eucope ploystyla. (After Kowalevsky.)
A. Blastosphere stage with hypoblast spheres becoming budded into the central cavity.
B. Planula stage with solid hypoblast.
C. Planula stage with a gastric cavity.
ep. epiblast; hy. hypoblast; al. gastric cavity.
Of the larval forms included in the above list a certain number are probably without affinities outside the group to which they belong. This is the case with the larvæ of the Myriapoda, the Crustacean larvæ, and with the larval forms of the Chordata. I shall leave these forms out of consideration.