Page:EB1911 - Volume 16.djvu/247

 of the coelomic vesicles which arise from the original gut diverticulum, it is impossible to resist the conclusion that there is affinity between the Echinoderm and Enteropneust phyla. Here we have a case like that of the Tunicata in which an affinity which is not evident from a study of the adult alone is revealed by a study of the young form. The other larva which recalls the Echinoderm type is the Actinotrocha of Phoronis (fig. 12), but the resemblance is not nearly so close, being confined to the presence of a postoral longitudinal band of cilia which is prolonged into arm-like processes.

The following groups have larvae which cannot be related to other larvae: the Porifera, Coelenterata, Turbellaria and Nemertea, Brachiopoda, Myriapoda, Insecta, Crustacea, Tunicata. We may shortly notice the larvae of the two latter.

In the Crustacea the larvae are highly peculiar and share, in a striking manner, certain of the important features of specialization presented by the adult, viz. the presence of a strong cuticle and of articulated appendages and the absence of cilia. They are remarkable among larvae for the number of stages which they pass through in attaining the adult state. However numerous these may be, they almost always have, when first set free from the egg, one of two forms, that of the nauplius (fig. 13, A) or that of the zoaea (fig. 13, B). The nauplius is found throughout the group and is the more important of the two; the zoaea is confined to the higher members, in some of which it merely forms a stage through which the larva, hatched as a nauplius, passes in its gradual development. The nauplius larva is of classic interest because its occurrence has enabled zoologists to determine with precision the position in the animal kingdom of a group, the Cirripedia, which was placed by the illustrious Cuvier among the Mollusca.

In the Tunicata the remarkable tadpole larva, the structure and development of which was first elucidated by the great Russian naturalist, A. Kowalevsky, possesses a similar interest to that of the nauplius larva of Cirripeds, and of the tornaria larva of the Enteropneusta, in that it pointed the way to the recognition of the affinities of the Tunicata, affinities which were entirely unsuspected till they were revealed by a study of the larvae.

With regard to the occurrence of larvae, three general statements may be made. (1) They are always associated with a small egg in which the amount of food yolk is not sufficient to enable the animal to complete its development in the embryonic state. (2) A free-swimming larva is usually found in cases in which the adult is attached to foreign objects. (3) A larval stage is, as a rule, associated with internal parasitism of the adult. The object gained by the occurrence of a larva in the two last cases is to enable the species to distribute itself over as wide an area as possible. It may further be asserted that land and fresh-water animals develop without a larval stage much more frequently than marine forms. This is probably partly due to the fact that the conditions of land and fresh-water life are not so favourable for the spread of a species over a wide area by means of simply-organized larvae as are those of marine life, and partly to the fact that, in the case of fresh-water forms at any rate, a feebly-swimming larva would be in danger of being swept out to sea by currents.

1. The association of larvae with small eggs. This is a true statement as far as it goes, but in some cases small eggs do not give rise to larvae, some special form of nutriment being provided by the parent, e.g. Mammalia, in which there is a uterine nutrition by means of a placenta; some Gastropoda (e.g. Helix waltoni, Bulimus), in which, though the ovum is not specially large, it floats in a large quantity of albumen at the expense of which the development is completed; some Lamellibranchiata (Cyclas, &c.), Echinodermata (many Ophiurids, &c.), &c., in which development takes place in a brood