OF THE PEOTOZOA. CILIATA. 299 



against the opaque background, and it becomes necessary to crush the 

 animalcule beneath the coveiing glass, so as to press out the green globules 

 which it contains, in order to bring the fusiform bodies into view. To these 

 bodies I propose to give the name of tricliocysts. 



" As long as the animalcule continues free from annoyance, the trichocysts 

 undergo no change ; but when subjected to external irritation, as occurs diu'ing 

 the diying away of the siuTounding water, or the aj^plication of acetic acid 

 or other chemical irritant, or the too forcible action of the compressor, they 

 become suddenly transformed into long filaments, which are projected fi'om 

 all parts of the suiface of the animalcule ; and it is these filaments which, 

 being mistaken for cilia by Cohn and Stein, gave rise to the erroneous views 

 just mentioned. 



'' The rapidity with which this remarkable change is efi'ected, joined \di\\ 

 the great minuteness and transparency of the object, renders it extremely 

 difficult to follow it ; and for a long time I could only satisfy myself of the 

 fact that the fusiform bodies were suddenly replaced by the projected fila- 

 ments. After continued observation, however, I at last succeeded in mt- 

 nessing the principal steps in the evolution of the filament. 



" It is not difficult, by rapidly criLshing the animalcule, to force out some 

 of the trichocysts in an unchanged state. If the eye be now fixed on one of 

 the isolated trichocysts, it will most probably be seen after the lapse of a few 

 seconds to become all at once changed with a jDeculiar jerk, as if by the sudden 

 release of some previous state of tension, into a little spherical body. In this 

 condition it will probably remain for two or three seconds longer, and then a 

 spiral filament wiU become rapidly evolved from the sphere, apparently by 

 the rupture of a membrane which had previoiLsly confined it, the filament 

 imrolling itself so quickly that the eye can scarcely follow it, until it ulti- 

 mately hes straight and rigid on the field of the microscope, looking like a 

 very fine and long acicular ciystal. 



" This remarkable body, when completely evolved, consists of two portions 

 — a rigid spiculum-like portion acutely pointed at one end, and continuous 

 at the opposite end mth the second portion, which is in the form of an ex- 

 cessively fine filiform appendage less than half the length of the spiculum : 

 this second portion is generally seen to be bent at an angle on the fii^st, and 

 is frequently more or less curved at the free end. The form of the evolved 

 trichocysts is best observed in such as have floated away towards the margin 

 of the di^op of water, and are there left diy by the evaporated fluid. In 

 many of them the filiform appendage was not visible ; and they then merely 

 presented the appearance of a simple, long, fusiform spiculum. 



'' The resemblance of the organs now described, to the well-known thread- 

 cells of the Polypes and of certain other loAver members of the animal king- 

 dom, is obvious. That they are entirely homologous, however, wdth these 

 bodies we can scarcely yet assert. Their origin, at least, appears to be 

 different ; for, if we admit the unicellular structiu-e of the Infusoria, we have 

 the trichocysts apparently developed in the substance of the ceU-wall, instead 

 of being produced in special ceUs, as we know to be the case with the thread- 

 ceUs of the Polypes." 



These structuiTS have also arrested the attention of Oscar Schmidt, 

 Leuckart, and Lachmann. The second-named observer surmised them to 

 be '' poison organs ; " and very probably they have a defensive pm^pose, for this 

 is suggested both by AUman's history of them, and by Lachmann's observa- 

 tions {op. cit. p. 126, in foot-note) " of similar, but much thicker corpuscles, 

 which presented a deceptive resemblance to the urticating organs of the 

 Campanularia;, in an animal li^ing as a parasite " upon individuals of that 



