PARASITIC WORMS.

205

becoming a new generation; the sexual organ has become the sexual animal.

Now as the individual development of the Cladonema, and other Medusæ similarly propagated, corresponds with the systematic series of the Medusa polypes, the only reasonable and credible explanation of the ontogenesis of those Medusa in which heterogenesis. occurs, is that, in them, the historical development of the genus has become fixed. Neither the egg nor the hen were created. Before the delicately tinted Medusa populated the primæval ocean in lonely splendour, the Medusa polypes on the constantly changing shores were the sole representatives of the still infant class. Why single genera, like the Hydractinia, remained strictly conservative while others in various degrees paid homage to progress, whether and how the struggle for existence and survival of the fittest were here concerned, it is certainly impossible to prove in the individual species. But the general impression is decisive, and also the circumstance that the theory is consistent with the facts.

The evolutionary history of the intestinal worms leads to the same reflections and results. These animals, widely differing in their structure, were either created in or with their hosts, or else they have become habituated to them in a natural and direct manner. We may surely disregard the third alternative, that they were led by an innate "obscure impulse." According to our doctrine, the worms now passing the whole or a portion of their lives as parasites on or in other organisms, are descended from free and independent animals, and the periods occurring in their development, during which parasitic life

is exchanged for independent phases, signifies a reversion taking place systematically in all individuals to the once permanent condition of their progenitors. Of the Trematoda or Flukes, and Cestoda or Tapeworms, belonging to the class of the Platelmintha Suctoria, the latter have diverged the most from their starting-point ; their adaptation to life within other animals has rendered the alimentary canal superfluous, and their generations and transformations hence point less to their progenitors than is the case with a number of other Trematoda, with which many anatomical characters prove them to be closely related. Both, moreover, share the characters of their class with the free-living Turbellaria. From such as these, that is to say, from forms approximate to the present Turbellaria, the Trematoda and Cestoda must be descended, and with this agrees the free roving phase. which the larva of the Fluke (Distomum) undergoes as the so-called Cercaria, and previously as a rotating spherical body.

Many of the ciliated Nematoids, or thread-worms, too, the division which includes the Ascarides among others, have in their infancy a stage of independent life, during which they cannot be distinguished from the infantine forms of their more numerous kindred, which never adopt a parasitic life, and chiefly inhabit the sea. The transition to parasitism, as recapitulated by ontogenesis, was nothing more than an extension to a new territory offering advantages of nutriment; and on this point it is highly instructive to compare the Nematodes with the systematic series of the leechlike Suctoria (Trematoda), so excellently described by Van Beneden. We here find all the transitions

PARASITIC CRUSTACEA.

207

from independent predatory genera to others occasionally parasitic, and again from these to others which on leaving the egg immediately attach themselves for life. Here, as elsewhere, parasitism seems an adaptation to new habitats, which is recorded in the biography of the individual with a reminiscence of the previous form.

The circumstances of the parasitic worms are repeated. by the parasitic Crustacea, as, moreover, a probably primordial form of the crab family is preserved in the metamorphoses of several orders of this large and diversified, though coherent class. The larva, which, it may safely be assumed, approximates closely to the primordial form, was at one time taken for an independent genus and received the name of Nauplius. Hence a Nauplius. phase is spoken of, which obtains especially among the lower Crustacea, the Copepoda, parasitical Crustacea and Cirripedes, and the remarkable Rhizopoda connected with them; but is not wanting in the highest order, the decapodous stalk-eyed crab. We shall later have to make acquaintance with the so-called curtailed development which among the crabs has been adopted by the decapods, and it was formerly supposed by all. Were this actually the case, we should still, by analogy, infer their connection with the other orders repeating the Nauplius phase in the course of their development; but it was a welcome

FIG. 16.

discovery of Fritz Müller's that a shrimp (Peneus) still begins its development as a Nauplius; whereas all the other members of the order, as far as they are known, leave the egg in the higher Zoea phase (p. 50). As of the hundreds of stalk-eyed crabs, scarcely a dozen have been hitherto examined as to their development, it will not be doubted that, with regard to the Nauplius phase, some resemble the Peneus of the Brazilian coast. But even were this case to prove unique in the order, it would suffice as a living witness of the connection

FIG. 17. Axolotl.

between the presence of the decapods and the primordial crabs. There can be no other view of this subject. The Nauplius phase in the development of the Peneus is either a shining testimony in favour of the doctrine of Descent, or a senseless paradox.

After what has gone before, the transformation of the Amphibians needs no elucidation. Their predecessors were water-breathers, whose form and mode of life are more faithfully preserved by the long-tailed Amphibians, the tritons, and salamanders, than by the frogs. In our tritons, sexual maturity not rarely commences in the larval state, hence in a phase which was definitive in the progenitors of the present genera. There is, indeed, one species, the Mexican Axolotl, which normally propagates itself during the larval phase. Auguste

FIG. 18. Amblystoma.

Dumeril's observation is highly interesting, that of the thousands of Axolotls that he bred at Paris, some few advanced beyond the grade of development hitherto known in them, ie. they lost their gills, changed the