550 ORDER HOLOTRICHA. 



extremity ; cilia of the oral groove longer than those of the general surface, 

 but not projecting beyond the margin of the undulating membrane ; cuticular 

 surface delicately striate longitudinally, clothed with short, even cilia ; a 

 single, fine, straight, hair-like seta, about equal in length to the greatest 

 breadth of the body, produced from the centre of the posterior extremity ; 

 contractile vesicle conspicuous, postero-terminal ; endoplast double, con- 

 sisting of two equal-sized spheroidal moieties, situated one behind the 

 other a little in advance of the centre of the thicker posterior body-half. 

 Length 1-250". HAB. Salt water. 



It was at first proposed to include this species in the preceding genus Lembus, 

 but in the presence of the various forms now referable to that generic group, 

 none of which possess the anterior or posterior appendages that characterize the 

 present type, it has been deemed desirable to institute a new generic title for its 

 reception. The mobile and exceedingly flexible finger-like anterior process of 

 Proboscella presents, at first sight, some resemblance to the peculiarly modified apical 

 extremity of Trachelocerca or Lacrymaria, though it is not, as in those types, 

 perforated by the oral aperture. Its function appears to be entirely tentative, and 

 when not in use is, in common with the undulating membrane, packed away out 

 of sight. The habits of this species closely resemble those of Cyclidium glaucoma 

 and Uronema marina, it assembling in similar social companies, and for the most 

 part remaining stationary with its undulatory membrane extended and adoral cilia 

 rapidly vibrated. Under any disturbing influence it darts hither and thither with great 

 rapidity, exhibiting on such occasions, in common with Lembus velifer, the faculty of 

 swimming either forwards or backwards with equal facility. The examples- upon 

 which this new genus is here instituted were obtained in abundance from sea water 

 with decaying vegetation brought from Brighton in the month of May 1874. 



It is only quite recently, February 1881, on referring to O. F. Miiller's original 

 figures and description of Vibrio verminus * to ascertain the extent of the resem- 

 blance subsisting between that form and Lembus velifer, suggested by Cohn, that 

 the author has been enabled to positively identify Miiller's organism with the type 

 now under consideration, such identification justifying the adoption of Miiller's 

 original specific title in the place of the new one of P. mucronatus that had been 

 previously decided on. Although in Miiller's illustrations of this species no trace 

 whatever is given of either the cilia, caudal seta, undulating membrane, or anterior 

 tactile appendage, the identity here maintained is substantially established with 

 reference not so much to their corresponding clavate outlines, which perfectly agree, 

 but to the fact that Miiller has clearly indicated in several of his figures the existence 

 of a subcentrally located vesicular structure, at once recognizable as representing the 

 peculiarly modified double nucleus or endoplast of the author's type, while a third 

 structure of apparently the same nature, as delineated by Miiller, corresponds in a 

 similar manner with the contractile vesicle. Any further doubts that might yet 

 remain are immediately disposed of by a reference to Miiller's original description, 

 which confirms the account of the structure and habits of this animalcule already 

 submitted, in so full and interesting a manner as to invite quotation in extenso. 

 This description runs as follows : 



" Vibrio linearis compressus, antice quam postice angustior. 



" Animalculum hyalinum teres, motu vero lentiore crystallinum compressum, seu membrana 

 crystallina elongata, postica quam antica parte latior, apice subtruncata, basi obtusata. Interanea 

 nulla perceptibilia ; in medio tantum vesiculse duse sphaericse distantes, ac tertia intra marginem 

 posticam observabilis ; raro quasi linea longitudinalis prospicitur. 



" Motus festinans antrorsum et retrorsum vacillatorius, antea parte aliquantum elevata." 



Miiller further testifies to having on rare occasions observed animalcules either 

 in the act of conjugation or dividing by longitudinal fission. The examples he 



* ' Animalcula Infusoria,' p. 57 and Tab. viii. figs. 1-6, 1786. 



