the green-coloured infusoria to swallow colouring matter. In the Euglena viridis, Ehr. (Cercaria viridis, Müll), I recognised distinctly the red eyes, but could never detect any other red point, however long it was left in the solution of carmine. The remarkable Vorticella versatilis or Ophrydium versat. Ehr. could never either be induced to take the colouring matter. I found this species in great numbers towards the end of October, forming gelatinous balls of an inch and a half in diameter; they were best adapted for experiment, when their size was about "This wonderful animal is of a beautiful green colour, paler, and more transparent near the edges: their interior consists of a granular matter, or of small round vesicles (?)

IV. On the Change of Form, and the Successions of Infusoria in Infusions.

It appears very probable, from the observations of Ehrenberg, that the genus Monas, and several of the allied genera, are not distinct animal forms, but merely the young state of the Kolpoda, Paramacia, &c., which are unable to reach maturity without a division like the Rhizomorpha and Byssus among the Fungi. This idea has been subsequently adopted by Dr Eschweiler. I have myself no continued observations on the primitive generation of the Infusoria, but for several summers the appearance mentioned by Eschweiler* has appeared to me very remarkable. I have seen monads and other small infusoria appear in infusions which disappeared in a few days, and gave place to other genera. Thus the water above mentioned as

containing the Hydatina, on the second day after being taken from the pool, formed a green crust on its surface, particularly near the margin of the glass in which it was kept. When I examined this green mass with the microscope on the 23d October, it swarmed with the Euglena viridis and Enchelys pulvisculus, which swam about with much activity. Some of the first species were seen in the act of expiring, and others quite dead. During this curious phenomenon the elongated form of the animals was changed into a globular. They died in whole masses, particularly near the margin of the vessel; the green crust then forming the so-called Priestley's matter, although it may arise

* Isis. 1831, Heft. iv. p. 403.

as well from other green Infusoria. Another animalcule seen in considerable quantity was the Volvox globator of different sizes. On the 27th October the green mass of dead infusoria was still present, and formed a pulverulent layer on the side of the glass turned towards the light. On the surface there was a pellicle which exhibited numbers of the Kerone pustulata of " in size. Some had a transverse fissure, as figured by Ehrenberg; and there were a few examples of Vorticella larva, Müller. On the 3d November, not only the Euglena, but also the Kerone pustulata, had entirely disappeared. A new set of organisms had sprung up; the dead Euglena still retained their green colour and rounded form, and lay on the sides of the glass; many of the green masses, however, had become brown and yellow, and had aggregated together. Here and there I saw a Paramecium, and more commonly the Vorticella larva, the interior of which was filled with dead Euglena. There were also a few of the Hydatina senta. The Vorticella larva gradually disappeared as the Hydatina increased; and the latter at last vanished, so that, on the 10th November, the water was quite destitute of infusoria. Similar successions, though not in the particular order of this example, I have seen on different occasions. At other times I have observed several infusoria cotemporaneously existing together, as the Monas, Vorticella, Trachelius, Kolpoda, Kerone, Paramœcium, all of which disappeared at once. Certainly these transient swarms of living beings within the compass of a single glass, is an object of very great interest. We are quite ignorant of its cause. At first sight it might seem to favour the idea of the generatio equivoca, but my belief in this hypothesis has been pretty well shaken by Ehrenberg's numerous observations, as well as by the few which I have been able to institute in the course of a single summer. Nowhere in nature do we see any animal arrive at once at its perfect state; there is always a sort of dormant state previous to their properly independent existence. The invisible animating power of the Creator every where in animated nature attaches itself to the state of the egg, and at this state of existence more especially, does he seem to breathe into living beings the breath of life. The dormant state of the ovum seems to be the transition from the empire of mind to that of matter. The old

maxim "Omne vivum ex ovo," is for me, as well as Ehrenberg, equally binding upon the infusorial tribes. I certainly think it very possible that there may be a generatio equivoca of eggs, but not of perfect animals; but our data are as yet quite insufficient for the demonstration of the question.

V. On the supposed Connection between the Vegetable and Animal Kingdoms.

It is very commonly an admitted fact, that vegetables and animals touch one another in their lower degrees-that they even pass into each other, that not only in one genus do we find both animals and plants, therefore species from the two kingdoms of nature which can be separated by no generic distinction, but that even a plant often produces an animal which, during its growth, again changes into a plant. I must admit that these notions were once very attractive to me; that the globules of the blood were simple monads, which were formed from the living animal matter, like the infusoria from dead animal matter; that the Ectosperma produced monads which disappeared after the short life of an hour, and grew up to Confervæ. I not merely believed this myself, but have publicly taught it as an acknowledged fact, till I took the microscope in my hand and experimented for myself. I have now perhaps fallen into the other extreme; for, certainly, I have never succeeded in observing the separation of the globules of the blood from the parenchyma, although I do not pretend to deny its possibility; nor have I at any time seen how animals change into plants; and, notwithstanding Spallanzani's often repeated experiments, doubt very much of the resurrection of Furcularia after having been dried up for more than a year. When Treviranus, Bory St Vincent, and others, have seen globules issue from the tubes of the Confervæ and swim freely about like infusory animalcules; when Edwards has seen Confervæ again spring from these globules after settling; when Unger and others have seen active globules, which he calls Monads, issue from the Ectosperma clavata; I do not doubt the fact of globules having issued from these cryptogamic plants, which sprung about in different directions: these motions seem voluntary, therefore they are animals; they are very small and round, therefore they are mo

nads. Here, therefore, a moving globule and a monad are considered as identically the same. In order, however, to establish this, they should have been seen to take in colouring matter, and to contain internal cavities, which Ehrenberg has done with regard to all the true monads. Ingenhous, Treviranus, Hornschuch and Nees Von Esenbeck, if I mistake not, have said that the Priestley's matter consists of dead infusoria, and that from this congealed vegetable mass, as it is called, Confervæ, Ulvæ, Tremellæ, nay even the Hypnum riparia, have all been formed. Here I do not doubt the fact; times without number have I seen the Priestley's matter take its origin from green infusoria, such as the Euglena viridis, and the green pellicle in pools and infusions very generally consists of the dead bodies of infusoria, but never have I seen plants formed from this green matter; it continued long fresh, and became afterwards yellow, brown, run together, and fell to the bottom. When Confervæ grew in the infusion, or in the midst of these masses, there was never a transition between the dead infusoria and the living plants. Whether these plants arose from the spontaneous development of globules, or from the presence of infinitely small seeds in the water when drawn, I will not pretend to determine. Still less can I believe in the observations of Wiegmann, that the Entomostraci, Cypris and Cyclops are generated in putrid water from Priestley's matter, and after their death are changed into confervæ. He even asserts that he has seen confervæ springing out of the feet and antennæ. It is very possible that these animals have arisen on confervæ, but not from them.

VI. Structure of the Cercaria.

The exact observations and inimitable drawings which have been given of the Cercaria by Nitsch, must be known to every one who has occupied himself with researches into the infusoria. Subsequently Baer has made some highly important observations on their formation, after Bojanus had called our attention to the subject. During last summer I had an opportunity of observing the living Cercaria in some water which had been drawn from a pool for the purpose of examining the infusoria, and which had been at rest in a glass. It contained also many

planorbes, and as I think the Pl. carinatus. On examining the water in a few days, I observed with the naked eye some very active animalcules, which I immediately recognised as cercariæ by their very rapid motions. Their numbers increased on the following days. My observations were afterwards interrupted. On closer examination the animal appeared to be the Cercaria Lemna, Muller (Encyclop. Method. Pl. 8. fig. 8. 12.), a species which, as Nitsch supposes, is probably identical with the Cercaria major (Beytr. zur Infusorienkunde, Tab. ii. fig. 1–8.), for it resembles it exactly; only I could detect none of the fine hairs on the tail, which, however, only appeared to Nitsch with very high powers, and with the field of vision half illuminated. I employed a power of 48. For a long time I was quite delighted with the singular movements of these Cercariæ, but I turned my attention principally to their internal structure, which is not sufficiently known.

At

The cercaria which I observed was about 1" in length. The tail and the body were nearly of the same thickness; the latter was longitudinally striated. The substance of the animal was homogeneous, transparent, and of a milk-white colour. the anterior extremity was the round extensile opening of the mouth, from which it appears to be surrounded with a wreath which was notched. This mouth is succeeded by a smaller very distinct œsophagus, between which and the mouth the intestinal canal seemed to be somewhat narrowed. I saw no trace of the forked organ noticed by both Nitsch and Baer; nor was the course of the intestinal canal here marked by any dark spot, but it appeared to me merely to resemble a broad band extending from the mouth to the anus. The ovaria were very conspicuous, placed on the two sides of the body. When the animal was moderately extended, they made several convolutions in the form of an S, from the two sides of the oesophagus to the cloaca. The convolutions were less distinct when the animal was much extended. The hinder extremity was very short and broad, where the ovaria formed two irregular masses. Their texture appeared granular, and had a beautiful appearance, their dark colour being contrasted with the transparency of the rest of the body. Their exit into the cloaca I could not distinguish. Their origin towards the anterior part of the body