590 ORDER HETEROTRICHA. 



and permanent conditions of the same structures as met with in the genus Spirosio- 

 mutn. The anterior or distal end of the lateral fringe now begins to retreat from its 

 position immediately beneath the peristome of the parent zooid, and descending fromi 

 thence gradually curls round to meet the lower or oral termination. The area 

 enclosed within the almost conjoined extremities of this supplementary lateral fringe 

 represents now the peristome-field of the newly developed zooid, and corresponds 

 closely in both form and size with tlie pre-existing anterior one. The anterior half 

 of the body, leaving the original adoral wreath, further elongates, and becoming 

 gradually thinner at its posterior extremity, permits at length of its entire separation 

 from the posterior half now bearing the new peristome, and swimming off selects a 

 new point of attachment. In this process of segmentation, as above detailed, the 

 usually band-like endoplast takes its share, while at an early date a second contractile 

 vesicle makes its appearance in close proximity to the newly-formed adoral fringe. 

 Various phases of the process of subdivision, as above described, commencing with 

 the first appearance of the second peristome, will be found at PI. XXX. Figs, 14, 22, 

 24, 25, and 26. 



The so-called muscular or myophan layer in the members of this genus is perhaps 

 more conspicuously developed than in any other representative group of the Infusoria, 

 taking the shape of transparent thread-like fibrillse, which form a single, superficial, 

 closely approximated layer, and extend in an unbroken longitudinal series from the 

 peristome-border to the posterior extremity of the body. It is through the aid of these 

 highly elastic elements that the rapid and characteristic metamorphoses of external 

 contour are produced. A second series of hyaline fibrillas, surrounding concentrically 

 the peristome-border and enclosed area, acts sphincter-wise in closing up this region 

 when the animalcule is contracted. By not a few authorities the special contractile 

 or muscular properties have been wrongly accredited to the intervening coloured 

 and granular elements of the cortex. The posterior and adherent termination of 

 various species of this genus is frequently represented as provided with a special 

 suctorial disc. Such, however, does not exist, this region being simply abnormally 

 soft and plastic, permitting its free adherence to any selected object, this adhesion 

 being somewhat further assisted in many instances by the protrusion of fine 

 pseudopod-like extensions from the substance of the inner parenchyma or endo- 

 plasm, as represented at PL XXX. Fig. 23. 



The new generic title of Salpistes has been proposed by Dr. Strethill Wright * 

 for the reception of those species of Stentor which secrete gelatinous sheaths ; the 

 development of this accessory structure appears, however, to be so uncertain among 

 individuals even of the same species, that it has not been considered desirable to 

 adopt it in this treatise. 



Stentor polymorphus, Miill. sp. Pl. XXIX. Figs, 10-20. 



Body of large dimensions, the expanded anterior end when fully- 

 extended equalling in diameter one-third of the total length, the otherwise 

 transparent cortical layer usually containing a rich layer of green chloro- 

 phyll-granules ; endoplast distinct, moniliform. Length when fully ex- 

 panded 1-20", in contraction 1-120". 



Hab. — Standing water among both living and decayed vegetation, 

 eminently social. 



Stein embodies with this species the Stentor Mullen of Ehrenberg, which he 

 pronounces to be merely a variety of S. polytnorphus without the more usually 

 enclosed green chlorophyll-granules. The Stentor polymorphus, as figured and 

 described by Claparede and Lachmann, in the second volume of their ' Iiltudes,' 

 is referred by the same authority to the species next described. Although pre- 



* ' Edinburgh Philosophical Transactions,' vol. x. 1858. 



