Provided the indigo and carmine employed for the purpose have been sufficiently levigated, nothing is easier than to demonstrate the presence of the stomachal vesicles; but to exhibit the central canal, and the tubes that communicate between it and the gastric sacculi, is a much more difficult task, and can only be done under very favourable circumstances. We were, in. deed, long sceptical concerning their existence; but after examining Professor Ehrenberg's pre parations of these structures, we Were ulti mately convinced of the accuracy of his views concerning them.
Whoever wishes to see the intestinal tract distinctly must examine it in large specimens of some of the following species, rnost of which are sufficiently common :—Chelodon cu eullulus, Trachelius ovum, Epistylis plicatilis, Vorticella chlorostigma, Vorticella convallaria, Opercularia articulate, or Stylonychia mytilus. On putting a little indigo into the water with some of these, it may be readily seen to enter their large mouths, and pass into their stomachs, from which it is again speedily ejected.
In the Monads and allied families the ali mentary apparatus consists of several distinct cells, from eight to twenty in number, but which are not all of them filled at the same time. When contracted they are quite invisi ble; yet sometimes, when filled with a clear fluid, they are to be distinguished under the form of minute transparent vesicles in the in terior of the animalcule. The mouth may sometimes be easily perceived under the form of a clear transparent spot, situated at the base of the proboscis, to and from which streams of water may be seen to proceed, bringing with them the materials for nourishment (fig. 14). In the interior of the body the nutri tive sacculi appear like so many little empty bags hanging from the mouth. The food of the Monads seems to consist entirely of par ticles of decaying matter.
Dental system.—A very remarkable dental apparatus was discovered by Ehrenberg to exist in some of these diminutive beings, their presence being recognised in several different species, viz. Euodon cucullus (Synonyme, Kol poda, Larodes cucullus), Nassula ornata, Nas sula elegans, Nassula aurea, Prorodon niveus, Prorodon compressus, and others. Both in their form and connexions these teeth are very remark able, presenting the appearance of a long slender cylinder or hollow cone, situated at the entrance of the mouth, around which they form a closely approximated series (fig. 15). These teeth are composed of a hard substance ; for when the soft parts of the animalcule are crushed between two plates of glass, they still remain distinctly visible, proving that they are of a denser texture than the rest of the body. Their number varies in different genera from sixteen to thirty, the former being the minimum and the latter the maximum yet observed. In animalcules thus provided with a dental appa ratus the pharynx seems to have little to do with the act of nutrition ; indeed it frequently happens that while the little creature vibrates its cilia to produce the currents that bring it food, its mouth is kept open and motionless, so that the materials that serve for its nourish ment pass through it unobstructed : but when larger morsels are to be swallowed, they are first seized and bruised by the dental apparatus.
In this case the buccal cylinder first of all expands in front to receive the morsel; it is then narrow posteriorly : but as the aliment passes onward it becomes contracted in front and dilates behind, so as to push the food towards the mouth. Sometimes, however, these movements can be witnessed without any large morsels of food being present in the dental cylinder. While the mouth is kept open, Monads and other animalcules may frequently be seen to enter it with facility as far as the intestine ; in which case the contraction of the dental circlet seems to serve to prevent its re turn back again, should it try to escape in this direction.
A very remarkable circumstance observable in these teeth is the rapid manner in which new sets are formed as often as the fissiparous habits of the animalcules render their repro duction necessary. This regeneration of whole sets of teeth, a phenomenon so unusual among other races of animals, is among these Infu soria a matter of every day occurrence, a new set being produced whenever spontaneous di vision occurs : nay, should the animalcule be mutilated so that only the hinder half of its body remains, we are assured by Ehrenberg that the missing portions will soon be repro duced, provided with a new mouth and circle of teeth exactly similar to their predecessors ; and when they spontaneously divide by trans verse fissure, a process which occupies but a short space of time, the hinder portion, when separated, is found to be provided with a mouth and set of teeth completely organised in every respect (1, fig. 16). Sometimes, in deed, they may be observed during this sepa ration of the adult animal into two young ones, and the progress of the developement of the wanting parts absolutely witnessed. Under such circumstances Ehrenberg states, that such is the rapidity of the process that the division of the body, and the formation of a set of twenty new teeth, may easily be accomplished in the space of a couple of hours, acrite animalcules it is almost needless to say that no muscular fibres are obvious, although their bodies are capable of various contortions, and some of their movements under the microscope are extremely brisk and active. Nevertheless, in some of theVorticellinw, (Vorticella, Stentor, Carchesium, Opercularia,) Ehrenberg consi ders that their presence has been detected, and has even assigned their direction, some being, as he asserts, longitudinal and others trans verse. In the stems or pedicles of Carchesium and Tintinnus this appearance of muscular fibre is more especially evident ; and when we consider the highly organised condition of the genera in question, there seems to be no physi ological reason for considering their existence improbable.