Read Ebook: The Cambridge natural history Vol. 02 (of 10) by Beddard Frank E Frank Evers Benham W B William Blaxland Gamble F W Frederick William Hartog Marcus Sheldon Lilian Harmer S F Sidney Frederic Editor Shipley A E Arthur Everett Sir Editor

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MYZOSTOMARIA

HIRUDINEA

GEPHYREA

SIPUNCULOIDEA . PRIAPULOIDEA . ECHIUROIDEA . EPITHETOSOMATOIDEA .

POLYZOA

ENTOPROCTA

PLATYHELMINTHES AND MESOZOA

F. W. GAMBLE, M.Sc.

Demonstrator and Assistant-Lecturer in Zoology in the Owens College, Manchester.

TURBELLARIA

The Turbellaria were so called by Ehrenberg on account of the cilia or vibratile processes with which these aquatic animals are covered, causing by their incessant action, tiny currents in the surrounding water. The ciliary covering distinguishes this free-living group from the parasitic Trematodes and Cestodes, some of which possess such an investment, but only during their early free larval stage, for the short period when they have left the parental host and are seeking another .

Some Turbellaria resemble Infusoria in their minute size, shape, and movements. Nevertheless they possess an organisation of considerable complexity. The fresh-water Planarians , abounding in ponds and streams, vary from a quarter to half an inch in length, and are elongated and flattened. Their body is soft, and progresses by a characteristic, even, gliding motion like a snail. The marine Planarians or Polyclads are usually broad and leaf-like, sometimes attaining a length of six inches, and swim or creep in a most graceful way. Land Planarians occur in this country , but far more abundantly in tropical and sub-tropical districts, in moist places, venturing abroad at night in pursuit of prey. They are elongated and cylindrical, in some cases measuring, when fully extended, a foot or more in length, and are often ornamented with brilliantly coloured, longitudinal bands.

The Cestodes or Tape-worms have undergone more profound modifications both in structure and in mode of development. They are all endoparasitic, and, with one exception , attain maturity solely within the alimentary canal of Vertebrates. In length they range from a few millimetres to several metres, but this great size is attained from the need for the rapid production and accumulation of enormous numbers of eggs. The "head" or "scolex" is attached to the mucous membrane of the host by suckers or hooks, but there is no mouth nor any certain trace of a digestive tract at any stage of the life-history of Cestodes. For nourishment they absorb, through the skin, the previously-digested food that bathes them. In a few Cestodes the body is simple and not divided into "proglottides" or generative segments, but in most cases it is jointed in such a way that the last segment is the oldest, and each contains a set of reproductive organs. The life-histories of Cestodes are most remarkable. The proglottides containing the eggs pass out of the final host along with the faeces and enter the intermediate host with the food. The larvae hatch, and boring their way into the blood-vessels, are carried by the circulation to various internal organs. Here they usually become "bladder-worms," and develop the "head" of the future sexual form. Then, if, as is usually the case, the intermediate host is preyed upon by the final host, the larval Cestodes enter the alimentary canal of the latter. The head of the larva alone survives digestion, and from it the mature worm is formed.

Of these three branches of the phylum Platyhelminthes, the Turbellaria possess features of special interest and importance. Not only do they furnish the explanation of the structure of the two parasitic groups , but they occupy the lowest position in the whole group of worms. There are reasons for thinking that this is the simplest group of bilateral animals which adopt the habit of creeping. The Turbellaria are most closely allied to that great extinct group from which they, the Nemertinea, Rotifera, and even the Annelids, offer increasingly convincing evidence of having been derived. Many questions relating to the affinities of, or the origin of organs in, the Annelids, resolve themselves into similar questions about the Turbellaria. For these reasons, this group is here dealt with at greater length than the others, the interest of which is of a more special nature.

NERVOUS SYSTEM.--The brain, which is enclosed in a tough capsule , is placed in front of the pharynx, but some distance behind the anterior margin of the body. It is of an oval shape, subdivided superficially into right and left halves by a shallow depression, and is provided in front with a pair of granular-looking appendages, composed of ganglion-cells from which numerous sensory nerves arise, supplying the eyes and anterior region. Posteriorly the brain gives rise to a chiefly motor, nervous sheath , which invests the body just within the musculature. This sheath is thickened along two ventral lines and two lateral lines , but is very slightly developed on the dorsal surface. Ganglion-cells occur on the course of the nerves, and are particularly large at the point of origin of the great motor nerves.

The "marginal groove" is a shallow depression of the epidermis lined by cilia, and containing the ducts of very numerous gland-cells. It runs almost parallel to the anterior margin of the body, a short distance from it, but we have no observations on its functions.

CLASSIFICATION, HABITS, AND STRUCTURE OF THE POLYCLADIDA.

The Polyclads were so called by Lang on account of the numerous primary branches of their intestine. They are free-living, purely marine Platyhelminthes, possessing multiple ovaries, distinct male and female genital pores , but no yolk-glands. The eggs are small, and in many cases give rise to a distinct larval form, known as "M?ller's larva" . The Polyclads, with one exception, fall into two sub-groups, Acotylea and Cotylea:--

Character. Acotylea. Cotylea.

Sucker A sucker absent. A sucker always present .

Mouth In the middle, or behind the In the middle, or in front of middle, of the ventral the middle, of the ventral surface. surface.

Pharynx More or less intricately Rarely folded. Usually folded. cylindrical or trumpet- shaped.

Tentacles A pair of dorsal tentacles A pair of marginal tentacles usually present. .

Development Usually direct. Larva when M?ller's larva present. present, not a typical Metamorphosis, however, M?ller's larva. extremely slight.

Fig. 8 shows that, starting with a member of each division, in which the mouth is almost in the middle of the ventral surface, and the brain and sense organs somewhat remote from the anterior end, we find in the Acotylea a series leading to an elongated form , in which the mouth, pharynx, and genital pores are far back near the hinder end of the body; while in the Cotylea the series leads similarly to the elongated Prosthiostomatidae, in which, however, the pharynx and external apertures are in the front part of the body. This view of the morphology of the Polyclads is due to Lang, and is based on the assumption that the more radially-constructed forms are the primitive ones.

CLASSIFICATION OF POLYCLADIDA.

ACOTYLEA.

Family. Genus. British Representatives.

HABITS OF POLYCLAD TURBELLARIA.--Polyclads are exclusively marine, and for the most part littoral, animals. Moreover, there is no evidence of their occurrence in those inland seas where certain marine animals have persisted under changed conditions. From half-tide mark down to 50 fathoms, some Polyclads probably occur on all coasts, but as to their relative abundance in different seas we have very little accurate information. The southern seas of Europe possess more individuals and species than the northern, and probably the maximum development of the group takes place on the coasts and coral islands of the tropics. No Polyclads have been taken below 60 fathoms; but their delicacy and inconspicuousness render this negative evidence of little value. Six truly pelagic forms, however, are known, and these are interesting on account of their wide distribution , and also from the distinct modifications they have undergone in relation to their pelagic existence.

LOCOMOTION.--Locomotion is generally performed by Polyclads at night when in search of food, and two methods, creeping and swimming, are usually employed--creeping by the cilia, aided possibly, as in the case of some Gasteropod Mollusca, by the longitudinal muscles of the ventral surface; and swimming, by undulations of the expanded margins of the body. In the former case the cilia work in a glandular secretion which bathes the body, and enables them to effect their purpose equally well on different substrata. The anterior region is generally lifted up, exploring the surroundings by the aid of the tentacles, which are here usually present. The rest of the body is closely appressed to the ground.

The sense organs of Polyclads have the form of tentacles, eyes, otocysts , and stiff tactile cilia. The solid dorsal tentacles of Planoceridae contrast strongly with the folded or pointed hollow processes of the Cotylea. The former are muscular and very contractile, and are placed near the brain some distance from the anterior end. The latter are outgrowths of the front margin of the body, and are sometimes provided superficially with olfactory pits and internally with eyes and intestinal coeca.

REPRODUCTION.--Although Polyclads are able to repair the result of injuries to a very considerable extent, they are not known to multiply asexually. The two processes are intimately associated, but, though probably all Turbellaria can regenerate certain lost parts, asexual reproduction only occurs sporadically.

Polyclads possess an undoubted mesoderm, which gives rise to the muscles, the pharyngeal fold, and the parenchyma. The ectoderm forms the epidermis, in the cells of which the rhabdites arise, apparently as so many condensed secretions. From the ectoderm the brain arises as two pairs of ingrowths, which fuse together, and from these the peripheral nervous system grows out. Three pigmented ectoderm cells give rise, by division, to the eyes--an unpaired cell to the cerebral group of eyes, and the other two to the marginal and tentacular groups. The copulatory organs apparently arise to a large extent as ingrowths from the ectoderm, from which the accessory glands are also formed. The endoderm forms the lining of the main-gut and its branches. The pharynx is developed as in Fig. 13, which shows that the "mouth" of the young larva does not correspond exactly with that of the adult . The salivary glands arise from ectoderm cells, which sink deeply into the parenchyma. The reproductive organs possibly arise by proliferation from the gut-cells . The change from the larva to the adult is gradual, the ciliary band being absorbed and the creeping mode of life adopted.

CLASSIFICATION OF TRICLADIDA.

PALUDICOLA.

Family. Genus and British Species.

MARICOLA.

TERRICOLA.

CLASSIFICATION OF RHABDOCOELIDA.

ACOELA.

Family. Genus and British species.

RHABDOCOELA.

ALLOEOCOELA.

TREMATODA

From the Turbellaria we now pass on to a consideration of the second great subdivision of the Platyhelminthes, the Trematodes or "flukes," of which the "liver-fluke" is the best known, since it is one of the most dangerous parasites that infest domestic animals.

TREMATODA MONOGENEA .

There are four subdivisions of the Monogenea:--

HABITS AND STRUCTURE OF ECTOPARASITIC TREMATODES.

The reproductive system is very similar to that of certain Rhabdocoels. An armed penis and the female genital duct open into a genital atrium, and this by a single aperture to the exterior. The fertilised ovum and yolk are enclosed in a stalked shell formed in the uterus.

The interest and importance of the Temnocephalidae lies in the fact that they are almost as much Turbellaria as Trematodes. In habits, in the character of the skin, the muscular, digestive, and reproductive systems, they find their nearest allies in Rhabdocoels . But in the excretory and nervous systems, the latter composed of two dorsal, two lateral, and two ventral trunks all connected together , they are Tristomid Trematodes. Thus they may fitly connect an account of the two great groups.

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