Page:EB1911 - Volume 21.djvu/737

Rh algal-frequenting and cryptic habits, the Turbellaria, though soft-bodied, are able to withstand the violence of the waves.

The anterior end in all Turbellaria is the site of the chief sense-organs, and in some forms (Proboscida) becomes transformed into an invaginable proboscis of highly tactile nature. Such forms lead naturally to the (q.v.).

Coloration.—The coloration of Planarians is of interest. The flattened marine forms are often brilliantly coloured on the dorsal surface, either uniformly or with some striking marginal band; or they may exhibit longitudinal bands of contrasting tints or a mottled appearance. The significance of these colours is not fully understood, but in some cases of sympathetic coloration the derivative function of the pigments is probably to aid cryptic resemblance. The terrestrial Planarians exhibit the most striking patterns in longitudinal striping and cross-bars which appear to have no relation to the environment of these essentially nocturnal animals. The fresh-water forms are colourless or dusky, often dark-brown, possibly in relation to the retention of heat, but in a number of both fresh-water and marine Planarians a green colour is present, constantly in some species, sporadically in others.

This green effect is due to the infection of the Planarian by a minute alga which multiplies in the tissues and may profoundly affect the habits and even the structure of its “host.” The planarian so affected acquires a heliotropic habit; it becomes gregarious and in extreme cases ceases to ingest solid food. In Convoluta roscoffensis the green cells have become indispensable. They function both as the nutritive and excretory organs of the Planarian, and the young animal cannot develop until it is infected and has acquired a supply of these green cells which become incorporated into its tissues (Gamble and Keeble [7]). Brown algal cells (Zooxanthellae) are known in other species of Convoluta.

Food.—The food of Turbellarians consists, in the smaller species, of diatoms, unicellular algae, microscopic animals and other Turbellarians; in the larger ones, of worms, mollusca and insects. The fine feeders capture their food chiefly at night by gulping down the minute organisms that settle or swim in their neighbourhood. The coarse feeders enclose their prey with a coating of slime and then proceed either to engulf it in their expansible mouth or to perforate it by their trumpet-like pharynx. The mouth is remarkably variable in position (fig. 2). In many flattened Planarians it is placed centrally on the ventral surface somewhat as in a jelly-fish. In the majority it is nearer the anterior end, but in a few remarkably elongate forms it occupies a position near the hinder end of the animal. In the cylindrical forms (Rhabdocoels) a similar variability in the position of the mouth is met with.

Anatomy.—The structure of the Turbellaria though greatly varied in detail, conforms to a single type of somatic organization which is transitory in the higher invertebrates. The sexual organs,

on the other hand, are founded on two or more types, and the astounding complications of these structures suggest that their evolution has been governed by quite other factors or combinations of factors than those that have guided the somatic evolution of the group.

The general structural characters are as follows. The body consists of a muscular envelope covered externally by a ciliated glandular epidermis and of an alimentary sac, cylindrical or branched, for which the mouth serves both as ingress and egress. Between this aproctous gut and the integument the body consists of a jelly-like, vacuolated mesenchyme made up of branched gland-cells, excretory cells, pigment- and muscle-cells. A space may be secondarily hollowed out around part of the gut; but no coelomic or true perivisceral cavity exists in the sense in which these terms are used in higher animals. A nervous system is present and consists of an anterior “brain” and of ramifying ganglionic trunks that are developed in relation to the muscular integument and to the sense-organs for the perception of light and pressure. No