GENUS DIDINIUM. 639 



but are subservient merely to the purpose of locomotion. It is perhaps desirable, in 

 recognition of this circumstance, that in conjunction with Mesodinium, it should be 

 separated as the representative of an independent family group. 



Didinium nasutum, Mull. sp. PL. XXXII. FIGS. 50-57. 



Body ovate or barrel-shaped, about twice as long as broad, rounded 

 posteriorly, the anterior border produced in a snout-like manner ; paren- 

 chyma coarsely granular, cuticular surface smooth ; the foremost ciliary 

 wreath developed close to the base of the snout-like anterior projection, 

 the hindermost girdle encircling the body at about a distance of one-third 

 of its total length from the posterior extremity ; proboscis slender, protru- 

 sible to a distance equalling the entire length of the body ; endoplast band- 

 like, curved ; contractile vesicle spherical, of large size, debouching upon the 

 anal aperture. Length 1-300". HAB. Pond water. 



The Vorticella nasuta of O. F. Miiller, which furnishes the type of both the genus 

 and species now under consideration, has recently been the subject of investigation 

 by Balbiani,* who has elicited details of high interest concerning both its structure and 

 life-history. By this authority it has been first recorded that the animalcule is able 

 to evert from its snout-like anterior extremity a proboscidiform organ of considerable 

 length, with which it seizes upon and sucks out the contents of other animalcules, 

 such as Paramczcium, in the same manner that an Acineta devours its living prey, or 

 transports it bodily, with the retraction of its proboscis, to its own interior. 

 Balbiani further ascribes to this form a tubular central digestive canal, presenting at 

 the pharyngeal or post-oral portion of its course a longitudinally striated aspect, which 

 is produced by the presence in its walls at this point of numerous rigid rod-like 

 filaments. These pharyngeal rods are, according to his observations, capable of 

 extrusion and are used by the animalcule for the purpose of paralyzing its prey. 

 From the account given of these elements, however, the author is inclined to 

 identify this rod-system as a whole with the pharyngeal rod-fascicles of Chilodon, 

 Prorodon, and other Hypotrichous and Holotrichous forms. A very regular proto- 

 plasmic circulation has been observed by Balbiani, flowing in a continuous stream, 

 immediately beneath the cuticle, towards the apical or oral pole, and returning along 

 the central axis formed by the digestive tract. 



With Didinitim nasutum must be identified the so-called Chytridium Steinii, 

 figured and described by Dr. Ernst Eberhard in the ' Osterprogramm der Realschule 

 zu Coburg' for 1862, as also the more recent Wagnerella cylindroconica, described, 

 with an accompanying woodcut, by Wladimir Alenitzin, of St. Petersburg, in the 

 ' Archiv f. Mikroskopische Anatomic,' Bd. x., 1874. Increase by transverse fission 

 is of very common occurrence in this type, such an approaching duplicative act 

 being always heralded by the accession to the body surface of two supplementary 

 ciliary girdles. In some instances spheroidal germ-like bodies were observed by 

 Balbiani occupying the place of the normal band- like endoplast, and through the 

 breaking up of which they were apparently produced. Subsequently these germs 

 were liberated as minute pyriform animalcules, bearing a single anterior circlet of 

 cilia, and corresponding closely with the adult state of the preceding genus Meso- 

 dinium. The growth of these germs into the perfect form has not as yet been 

 traced. The present author has on one occasion obtained this interesting animalcule 

 in the neighbourhood of Stoke Newington, London. A reference to the homo- 

 plastic resemblance that subsists between Didinium and the embryos of certain 

 metazoic organisms will be found at pages 576 and 577. 



* " Sur le Didinium nasutum," ' Archives de Zoologie Experimentale,' 1873. 



