Fresh-water Worms--Planarians--Hydra--Polyzoa--Rotifers--Chætonotus--
Water-Bears.
The fresh-water worms form a large and well-defined group, and a few
words regarding them may be useful.
They are very common, and very difficult to find information about,
most of the work relating to them having been done in Germany. At the
same time, they are so highly organised and so transparent that the
process of their life-history may be easily followed.
One large group has the peculiarity of multiplying by division, the
last joints or segments being devoted to the formation of the new
individual. At one time of the year the ordinary sexual process of
reproduction takes the place of this method, and each worm is then
surrounded by a belt such as may be seen in the common earthworm under
similar conditions. Further information on this subject is greatly
needed.
The type is the common _Naïs_, which has a body of thirty segments or
more, two eye-specks on the head, and a double row of bristles along
the back; whilst below, each segment carries strong hooked bristles,
nearly buried in the body, by means of which the worm crawls. Inside
the mouth is a large proboscis, which can be protruded, and this leads
into a stomach which is merely an enlargement of the intestine which
succeeds it. The circulation of the blood (which is colourless) can be
easily watched. It begins at the tail with a contraction of the dorsal
vessel, passes up to the head, and then down below the intestine to the
tail again. The intestine is ciliated inside, and it is by a current
of water carried into the intestine by these cilia that the blood is
aërated.
In the next genus, _Dero_, this is clearly seen, for the tail (Plate
XIV. Fig. 1) is opened out into a wide shield, from which rise four,
six, or even eight finger-like processes. These parts are all ciliated,
and contain a network of blood-vessels. The worm lives in a case which
it builds in the mud, and the way to find it is to put some of the mud
into a glass beaker with water, and allow it to stand. If there be
members of this family in it, their tails will be seen protruding above
the water. Pour out the mud sharply, fill up with water, and allow the
dirt to subside, and the worms may then be made to leave their cases by
pressure by a camel hair pencil on the lower end of the tube, and may
be caught with the dipping tube and placed in the live-box. They have
no eyes, otherwise the general outline of the body closely resembles
that of Naïs.
_Slavína_ (Fig. 2) has a row of touch-organs, like pimples, round
each segment, and is a dirty looking creature, with an inordinately
long first pair of bristles, but this reaches its acme in _Pristina_
(Fig. 3) (sometimes, though wrongly, called _Stylaria_) _parasita_,
which has three long sets of bristles upon the back, and keeps these in
constant wing-like motion. The true _Stylaria_ has a long trunk, set
right in the head, and tubular (Fig. 6); it grows to a considerable
length, and when in the stage of fission it is very funny to see
the two proboscides waving about, one on the middle, as well as the
original one at the head. There is also a form with a shorter proboscis
of the same kind.
_Bohemilla_ has a tremendous array of saw-like bristles upon the back,
whilst _Chætogaster_ has none at all in this position, and few below.
_Æolosoma_ has merely tufts of hair instead of bristles, and swims
freely. It is easily recognised by the red, yellow, or green pigment
spots in its skin, and by the ciliated head. Rarest of all the family
is the one which connects it with the ordinary _Tubifex_, the red worm
which lives in masses in the mud of brooks and ponds, the waving tails
protruding above the water, and being instantly withdrawn when a foot
is stamped upon the bank. Their Naid cousin is _Naidium_, and has red
blood, but multiplies by fission, which Tubifex does not.
Another group of worms is the _Planarians_, small leech-like worms,
black, white, or brown, which are rarely absent from a gathering. The
would-be investigator will find in them an abundant field for work, as
they are but very imperfectly known or studied.
The great enemy of all these worms is the _Hydra_, a good idea of
which may be formed from Plate IX. Fig. 13. There are three species,
all of which are fairly common. They capture their prey in exactly the
same way as sea-anemones and the marine hydroid forms, so numerous and
varied.
Nor must we omit to notice the exquisitely beautiful Polyzoa, such
as _Lophopus_ (Plate XIV. Fig. 4), with its ciliated tentacles and
transparent social home; _Fredericella_ (Fig. 5), with its graceful
stems, and their still more graceful inhabitants; and the wonderful
_Cristatella_, whose colonies form bodies which crawl over the stems
of water plants. But for grace, beauty, and variety, the Rotifers
assuredly outshine all their fellow inhabitants of our ponds and
streams.
We can only take a few types, and of all these the most common is the
common Rotifer (Plate IX. Fig. 10). It is there shown in the act of
swimming, but it can withdraw its “wheels” and creep like a leech,
protruding its foot as it does so. It is distinguished by the two
eye-spots on the proboscis from _Philodina_, in which they are on the
breast, and _Callidina_, which has none. When at ease in its mind, the
animal protrudes its wheels, and by their action draws in particles
of food, these passing down to the incessantly moving jaws, which act
like a mill and crush the food before it passes on to be digested. The
movement of the jaws may even be seen in the young Rotifer whilst still
in the egg within the body of the parent, and as the egg reaches its
full development other eggs again are visible within it, so that we may
have three generations in one individual. The males of most of the
Rotifera are unknown. Those that are known are very lowly organised,
having only the ciliary wreath and the reproductive organs, and are
only found at certain seasons of the year. For the remainder of the
time parthenogenesis is the rule, just as among the Aphides. We select
a few individuals for illustration as types. Those who wish to pursue
this study further must be referred to the monumental work of Hudson
and Gosse.
The common Rotifer, already referred to, may be taken as the type
of the Bdelloida, or leech-like class, so called from their mode of
“looping” themselves along. The group is a comparatively small one in
comparison with the next, the Ploïma, or free-swimmers. We can only
select from the vast variety a few species, first of those classed
as illoricated, from their being without a _loríca_, or case, and
then of the loricated, which possess it. A very large and common
form is _Hydátina_ (Plate XIV. Fig. 7), which lives by choice in the
reddish pools of water found often by the roadside. It shows the whole
organisation of the class magnificently; the ciliary wreath on the
head, with the striped muscles which draw the latter back, the powerful
jaws, the digestive canal with its crop and intestine, the ovary
with the developing eggs, the water-vascular system with the curious
vibratile tags, and finally, the cloaca, which receives the waste of
the body and expels it at intervals.
Another form, also common, especially in clear water, is _Synchæta_
(Fig. 8), very much like a kite or peg-top in shape, which has the
power of attaching itself by a glutinous thread, and spinning round at
a tremendous rate. Then there is the gigantic _Asplanchna_ (Fig. 9),
which has no opening below, so that the waste must be discharged by the
mouth; and curious _Sacculus_, which gorges itself with chlorophyll
until it looks like a green bag with a string round it, but clear and
sparkling. Of the _Notommatæ_ there is a whole host, but we can only
mention the beautiful _N. Aurita_ (Fig. 10), with an eye of a beautiful
violet colour, composed of several spherules massed together, and two
curious ear-like processes on the head, from which it takes its name.
Some of the Ploïma have powers of leaping which must be noticed. The
_Triarthra_ (Fig. 11) has three arms, or what we may call such, which
it can stretch out suddenly and leap to a considerable distance, whilst
in _Polyarthra_ the arms become a whole cluster of broad saw-like
bristles.
We pass on to note a few species of the mail-clad or loricated
Rotifers, chief among which the great _Euchlanis_ (Fig. 12), a
noble-looking fellow, calls for our attention, his great size rendering
him easily visible to the naked eye. It is difficult to avoid using the
masculine gender, but, of course, all those figured and described are
of the gentler sex. _Salpina_, too (Fig. 14), with its box-like lorica,
armed with spines at each of the upper angles, and having three below,
is quite easily recognised, and very common. _Brachionus_ (Fig. 13)
has a shield-shaped case, well furnished with spines, symmetrically
arranged at the top, and an opening below for the flexible wrinkled
tail, like the trunk of an elephant. _Pterodina_ (Fig. 15) has a
similar tail, but a round case, and the head is much more like that
of the common Rotifer when extended. _Anuræa_ (Fig. 16), on the other
hand, has no tail, and its case is shaped like a butcher’s tray, with
a handle at each corner. _Dinocharis_ (Fig. 17) has a roof-like case,
with long spines on the root of the tail, and a forked stiff foot.
_Noteus_ (Fig. 18) is much like Pterodina, except in its foot, which
more nearly resembles that of Dinocharis.
The list might be indefinitely extended, but sufficient has probably
been said to enable the tyro to find his bearings in this large,
beautiful, and interesting class.
We pass on to notice in conclusion two or three of the fixed forms,
of which a beautiful example is the _Melicerta ringens_ (Plate IX.
Fig. 7), whose building operations have a never-ending charm. Particles
of débris are accumulated in a curious little cavity in the chin, in
which they are whirled round, and mixed with a secretion which binds
them together, and when a brick is made the head is bent down and the
brick applied to the desired spot with mathematical regularity. By
supplying fine particles of innocuous colouring matters, the Melicerta
may be made to build a variegated case. The most remarkable specimen
known is the one figured in Hudson and Gosse’s work, which was found
by the present writer in a specimen of water from which he had already
obtained five-and-twenty species of various kinds of Rotifer; the water
was collected by an inexperienced person, and there was only a pint
of it. It had, moreover, been kept for three weeks, and the moral of
that is, to preserve one’s gatherings, and keep an aquarium into which
they may be poured when done with for the moment. New forms will often
develop with startling rapidity, their eggs having been present in the
original gathering. The young form of Melicerta, shown in Plate XIV.
Fig. 20, is strangely unlike its mother, and much more nearly resembles
its father.
Another group of extreme beauty is the Flosculariæ (Fig. 19), several
species of which are very common. They will be easily known by their
appearance, which resembles a shaving brush when closed; whilst, when
opening, the shaving brush resembles a cloud of delicate shimmering
threads, which at last stand out straight, radiating all round the head
of the creature, and forming the trap by means of which it catches its
prey. Finally, there is the lovely _Stephanoceros_ (not, unfortunately,
very common), with its five symmetrically placed and gracefully curved
arms, perhaps the most lovely of all Rotifers, with its exquisitely
transparent body, sparkling with masses of green and golden brown.
He who finds this has a treasure indeed, and will be encouraged to
prosecute his studies in this “Fairyland of Microscopy.”
Two irregular forms call for a word of remark. The first is
_Chætonotus_ (Plate XIII. Fig. 27), which stands on the borderland of
the Infusoria and the Rotifers, neglected as a rule by the students
of both; and the second the _Tardigrada_ (Plate XIV. Fig. 21), or
water-bears, which have feet like those of the red wriggling larva of
_Chironomus_, whose silky tubes are common enough on submerged walls
and on the stems of plants, these feet consisting of a mass of radially
arranged hooklets, which can be protruded or withdrawn at will; whilst
the head of the water-bear is far more like that of a louse, pointed
and hard, and suited for burrowing about, as the animal does, among the
rubbish at the bottom of the bottle. Both the genera just referred to
will repay careful study, as little is known of their life-history or
development.
A few words must be devoted, in conclusion, to the Entomostraca,
those shrimp-like animals which, like their marine relatives, act as
scavengers to the community. Fig. 22 is a portrait of _Cypris_, a not
very handsome form, but one very commonly found. Its shell is opaque,
so that the internal organs are difficult to observe. Far different
in this respect is the beautiful _Daphnia_, the water-flea _par
excellence_, whose carapace is of crystalline clearness, so that every
movement of every one of the internal organs may be followed with the
greatest facility. There are many species of the genus, and some of
them are very common, so that the opportunity of examining these lovely
objects is easily obtained. Plate XIV. Fig. 23, shows the most common
of all the class under notice, the _Cyclops_, so named from the fact
that, like the fabled giants of classical literature, it has a single
eye in the middle of its forehead. It is often loaded with Infusoria,
especially Vorticella and Epistylis, already described, to such an
extent that its movements are greatly hampered.
XII.
FIG.
1. Tubercle, Sun-star
2. Zoophyte, Gemellaria
3. Cuttle bone
4. Plate of ditto from above
5. Zoophyte, Antennularia
6. Pedicellaria, skin of Starfish
7. Shell, Foraminifer
8. Snake-star, disc from below
9. Pedicellaria, Echinus
10. Wing-case, Weevil
11. Coralline
12. Spine, Echinus
13. Foraminifer, Polystomella
14. Do. Truncatulina
15. Do. Polymorphina
16. Do. Miliolina
17. Gold dust, with quartz
18. Foraminifer, Lagena vulgaris
19. Pouches, Skin of Rat’s tail
20. Foraminifer, Biloculina ringens
21. Ore, Copper
22. Zoophyte, Membranipora pilosa
23. Human skin, injected
24. Coal, Longitudinal section
25. Do. Transverse section
26. Lung, Frog
[Illustration: XII.]
We have not space to figure more of these creatures, but other forms
will be found not inferior in interest to those mentioned. The most
curious of all are those which earn a dishonest and lazy living by
attaching themselves to the bodies of other and larger animals, chiefly
fish. One of the largest is the _Argulus_, the bane of aquarium
keepers, which is of considerable size, and attacks gold-fish, and in
fact almost any fish to which it can obtain access.
The gills of the stickleback will furnish examples of the curious
_Ergasilus_, which consists chiefly of an enormous pair of hooks and
two long egg-bags, the latter, in varying form, being carried by many
of the Entomostraca.
Upon the fins of the same fish will be found the remarkable
_Gyrodactylus_, a worm-like animal which attaches itself by a large
umbrella-like foot, in the centre of which are two huge claws. The
head is split down the middle for some distance. We may mention, in
concluding our notice of the external and involuntary guests of the
unlucky stickleback, that its skin is usually frequented by hosts
of the Trichodina described in the last chapter. Of the internal
parasites, want of space forbids us to speak.