Pulmonata

“ Gastropods.
The shell of the Pulmonata, though always light and delicate, is in many cases a well-developed spiral “house” into which the creature can withdraw itself; and, although the foot possesses no operculum, yet in Helix the aperture of the shell is closed in the winter by a complete lid, the “hybernaculum” more or less calcareous in nature, which is secreted by the foot. In Clausilia a peculiar modification of this lid exists permanently in the adult, attached by an elastic stalk to the mouth of the shell, and known as the “clausilium.” ”

“ In connection with the plants above noticed, it is interesting to observe that at Green’s Creek, at Pakenham Mills, at Montreal, and at Clarenceville on Lake Champlain, species of Canadian Pulmonata have been found in deposits of the same age with those containing the plants. The species which have been noticed belong to the genera Lymnea and Planorbis. ”

“ At the opposite extreme to Actaeon stand forms like Siphonaria and Gadinia, which are probably close links with the Pulmonata (p. 19) . The generative system of the whole group, which is, as in the Basommatophora, of the hermaphrodite type, without mutual fecundation, is another link of connexion with the Pulmonata. The respiratory organs present the most varied forms, sometimes consisting of one ctenidium (never two) , sometimes of secondary branchiae, variously placed, while sometimes no special organ exists. ”

“ For, of course, flowers do secure a mate, and they are aided in this enterprise by the most formidable array of helpers, one might almost call them conspirators. The chief of these are insects, thousands of different kinds of which are constant flower visitors. Some of the smaller birds, and even snails, also help flowers to meet their mates. The wind, too, bears pollen through the air to some expectant bride-to-be. And, finally, in the water, by a series of acts the like of which no one could improve for cunning, the cross-fertilization of certain aquatic plants is consummated. ”

“ Although with the Mollusca sexual selection does not seem to have come into play; yet many univalve and bivalve shells, such as volutes, cones, scallops, &c., are beautifully coloured and shaped. The colours do not appear in most cases to be of any use as a protection; they are probably the direct result, as in the lowest classes, of the nature of the tissues; the patterns and the sculpture of the shell depending on its manner of growth. The amount of light seems to a certain extent to be influential ”

“ This antenna, when touched, transmits a sensation or vibration to a certain membrane which is instantly ruptured; this sets free a spring by which the pollen-mass is shot forth, like an arrow, in the right direction, and adheres by its viscid extremity to the back of the bee. The pollen-mass of the male plant (for the sexes are separate in this orchid) is thus carried to the flower of the female plant, where it is brought into contact with the stigma, which is viscid enough to break certain elastic threads, and retaining the pollen, fertilisation is effected. ”

“ The water-beetles, of which there are many species, remain in the water all day, but toward evening come upon the ground and fly about, then plunge themselves again in the water at the approach of the rising sun. The larvæ of these insects are entirely aquatic, but when the time of their pupa state arrives, they take to the earth, where they make a spherical case; so that these insects are aquatic in the larva, terrestrial in the pupa, and amphibious in the imago state. ”

“ The plants have collected stores of organic matter, and their minute germs, along with microscopic species, are floating everywhere in the sea. Nay, there may be abundant examples of those Amœba-like germs of aquatic plants, simulating for a time the life of the animal, and then returning into the circle of vegetable life. In these some might see precursors of the Protozoa, though they are probably rather prophetic analogues than blood relations. The plant has fulfilled its function as far as the waters are concerned, and now arises the opportunity for the animal. ”

“ The tube below is larger than the first flower for about two-thirds its length, when it suddenly contracts, and if we cut it open we find the four anthers secreted near the wide base of the tube. What does it mean, this riddle of the bluets? For hundreds of years it puzzled the early botanists, only finally to be solved by Darwin. This is simply the little plan which the Houstonia has perfected to insure its cross-fertilization by an insect, to compel an insect to carry its pollen from one flower and deposit it upon the stigma of another. ”

“ There are many other genera and species of fresh-water polyzoa besides the Plumatella repens, and they [127] are found attached to sticks, stones, or leaves, generally to the under surface of the latter. They are all objects of great interest and beauty, which, whatever their diversity, conform sufficiently to one type that the student who has observed one, will easily recognise the zoological position of another. They should be viewed by transmitted and by dark-ground illumination, which produces very beautiful effects. ”

“ There can hardly be a doubt that the relative length of these organs is an adaptation for the safe transportal by insects of the pollen from the one form to the other. The exceptional cases in which these organs do not stand exactly on a level in the two forms may probably be explained by the manner in which the flowers are visited. With most of the species, if there is any difference in the size of the stigma of the two forms, that of the long-styled, whatever its shape may be, is larger than that of the short-styled. ”

“ We thus see that the turning down of the distal portion of the labellum, by which a temporary landing-place and an open door are afforded,—the upturned labellum, by which the flower is made tubular so that insects are compelled to crawl close by the stigmatic surface,—the pollen readily cohering to any object, and standing in friable pillars protected from the wind,—and, lastly, the large masses of pollen above and below that layer of grains, the tubes of which alone penetrate the edge of the stigma,—are all co-ordinated structures, far from useless ”

“ Since it has been shewn that the larva or caterpillar is really the moth, crawling, eating, and spinning, under the form of the worm, and that the pupa is only the moth swathed up, it is clear that they are not three beings, but that the same individual feels, tastes, sees, and acts by different organs, at different periods of its life, having sensations and wants at one time, which it has not at another ”

“ Some specimens are of a dull purple, others of a rich yellow hue; and I refer to the species as an interesting example of the influence of light on the colour of marine plants. The yellow specimens are exposed to the sun's rays, the purple ones are such as have grown in deep shadow. The species is the Laurencia pinnatifida of botanists.
Turning from the hidden clefts, we explore the deep pools that lie between the ledges. High wading–boots are necessary for this purpose, as we have to work in the water. ”

“ Many other species allied to the above possess similar devices, with slight variations; and there is still another group whose structure is distinctly adjusted to the tongues of insects—adaptations not merely of position of pollen masses, but even to the extent of a special modification in the entrance to the flower and the shape of the sticky gland, by which it may more securely adhere to that sipping member. ”

“ I allude to this fact because botanists in speaking of the fertilisation of various flowers, often refer to the wind or to insects as if the alternative were indifferent. This view, according to my experience, is entirely erroneous. When the wind is the agent in carrying pollen, either from one sex to the other, or from hermaphrodite to hermaphrodite, we can recognise structure as manifestly adapted to its action as to that of insects when these are the carriers. ”

“ The lower down the luckless insect gets the more certain is its death, and, to clinch matters, there is a tiny pool at the bottom where it is not only drowned, but, due to the composition of the mixture in the pool, digested. Only a very few plants can do this; only a minute fraction of the world’s vegetation can digest animal matter. ”

“ The pulvinus is developed imperfectly and to an extremely variable degree, so that apparently it is tending towards abortion. No such case has hitherto, we believe, been described. It is coloured green from its cells containing chlorophyll; and it is seated nearly in the middle of the petiole, instead of at the upper end as in all the other species. The nocturnal movement is effected partly by its aid, and partly by the growth of the upper part of the petiole as in the case of plants destitute of a pulvinus. ”

“ Here, then, we find an adaptation of certain parts of the moth's body in relation to the fertilization of the flower, but in this case it is as much in the interest of the moth as of the plant. By carrying the pollen to the stigma the moths secure the development of the ovules, which serve their offspring as food, so that we have here to do with a peculiar form of care for offspring, which is not more remarkable than many other kinds of brood-care in insects, such as ants, bees, Sphex-wasps, ichneumon-flies, and gall-flies. ”

“ Instead of the first change occurring in the female organs we may suppose that the male ones first varied, so that some individuals produced a larger supply of pollen. This would be beneficial under certain circumstances, such as a change in the nature of the insects which visited the flowers, or in their becoming more anemophilous, for such plants require an enormous quantity of pollen. The increased action of the male organs would tend to affect through compensation the female organs of the same flower; and the final result would be that the species would consist of males and hermaphrodites. ”

“ Of the organisms resulting from the aggregation of such creatures, we see that while some, as the Foraminifera, assume a certain definiteness of form, in their shells at least, others, as the Sponges, are very irregular. The Zoophytes and the Polyzoa are compound organisms, most of which have a mode of growth not more determinate than that of plants. But among the higher animals, we find not only that the mature shape of each species is very definite, but that the individuals of each species differ little in size. A parallel increase of contrast is seen in chemical composition. ”

“ Biology.—The zoaria of the species of Plumatella are found firmly attached to stones, bricks, logs of wood, sticks, floating seeds, the stems and roots of water-plants, and occasionally to the shells of molluscs such as Vivipara and Unio. Some species shun the light, but all are apparently confined to shallow water. ”

“ Among the remaining groups of algae, and containing by far the greatest portion of this class, are a number of orders and families comprising isogamous and heterogamous forms clearly related in various ways to one another, by vegetative structure and similar life-histories, but offering a wide range of variation in the form and habits of the sexual cells. And it is among these algae that we may find material upon which to base general conclusions on the sexual evolution of plants. ”

“ The true Zoophytes have all, more or less, the plant-like form, and they readily separate into two great classes, namely, the Anthozoa, or flower-life, and the Polyzoa, or many-life, in which the individuals are associated together in numbers. ”

“ Other instances are furnished by groups in which, though the nutriment is not abundant, the cost of living is almost unappreciable. Among the Cœlenterata there are the Hydroid Polyps, simple and compound; and among the Mollusca we have various types of Ascidians, fixed and floating, Botryllidæ and Salpæ.
But now from these low animals in which sexless reproduction, and continued multiplication of somatic cells, is common, and one class of which is named "zoophytes," because its form of life simulates that of plants, let us pass to plants themselves. ”

“ Their organs of vegetative life attain a disproportionate development. On the other hand, what are termed the "organs of relation" present a wide range of variation, as may be imagined from the fact that while some of the simplest representatives of the Mollusca consist of mere motionless sacs or bags, containing organs of digestion, respiration, circulation, and generation, its more complex forms are active predatory creatures, endowed with remarkable and varied powers of locomotion, and with sense-organs as keen and as highly developed as those of insects. ”

“ The exciting influence, therefore, which is transmitted from [page 266] one part of the leaf to another must be different from that which actually induces aggregation. The process does not depend on the glands secreting more copiously than they did before; and is independent of the inflection of the tentacles. It continues as long as the tentacles remain inflected, and as soon as these are fully re-expanded, the little masses of protoplasm are all redissolved; the cells becoming filled with homogeneous purple fluid, as they were before the leaf was excited. ”

“ He who will carefully examine the flowers of orchids for himself will not deny the existence of the above series of gradations—from a mass of pollen-grains merely tied together by threads, with the stigma differing but little from that of the ordinary flowers, to a highly complex pollinium, admirably adapted for transportal by insects; nor will he deny that all the gradations in the several species are admirably adapted in relation to the general structure of each flower for its fertilisation by different insects. ”