OF THE PROTOZOA. CILIATA. 283 



cylindrical short tube, with a serrate edge and longitudinally striated, is re- 

 presented by Stein to project from the opening of the sheath. It is, he adds, 

 separable above into two lips, which close when the animal retracts itself. 



It is not very unusual to meet with sheaths occupied by two animalcules, 

 — a cii'cumstance due to the act of self- division (XXVII. 10 ; XXVIII. 19). 

 In a few instances also, one, two, or more small young individuals lie free 

 within the sheath of the parent, e. g. Lagenophrys (XXX. 29, 34). The 

 sheath is always a product secreted from the animalcule, and first makes its 

 appearance around its base as a soft, homogeneous, colourless, jelly-like matter. 

 Diuing the process of its formation, the animal preserves a contracted state, 

 which diminishes, however, as the excreted layer advances, and ceases on its 

 completion ; and since each genus has a characteristic outline, as well in the 

 contracted as in the expanded condition, the sheath acquires also its special 

 character only. More or less of the posterior extremity is concerned in ex- 

 creting the formative matter ; but this having adhered to the anterior part 

 whilst in a contracted state, becomes di'awn forward by the progressive 

 elongation of the entire body, until at length, on full expansion taking 

 place, the connexion is broken and the sheath acquires a free edge. So soon 

 as excreted, the gelatinous layer proceeds to solidify, and simultaneously to 

 contract itself in thickness, so as to form a membrane, which, on its subse- 

 quent detachment from the fore part of the animal, forms a loosely-investing 

 case aroimd it. This description of the construction of the sheath applies to 

 all those genera where the animal is fixed at the bottom ; but in the instance 

 of Lagenophrys, where it is suspended from the constricted orifice of the case 

 by its peristom, some other plan of formation must be presumed, concerning 

 which, however, we have as yet, unfortunately, no direct observation to teach 

 us. In several species, as Goihurnia imberbis, the sheath not merely acquires 

 a parchment-Kke firmness, but also a decided colour — mostly yellow at first, 

 afterwards a rusty red. 



Dr. StrethiU Wright, of Edinburgh, has kindly sent us some notes on the 

 intimate structure of the sheath of Lagotia ; and doubtless they hold good to 

 a greater or less extent, so far as they represent general facts, in the case of 

 sheaths of other Oplirydina. He wiites — " The tube consists of yellowish 

 chitine, lined with a layer of dark-green sarcode of varying thickness (which, 

 I believe, secretes the chitine), and covered externally by a much thinner 

 layer of matter, which appears to be equivalent to the ^ coUetoderm ' of the 

 Hydroidce:' This structure is illustrated by figs. 12 and 13, PI. XXXI. 

 The following accoimt applies specially to the sheath of Lagotia (XXXI. 7, 8, 

 12, 13), which presents a series of rings, apparently spiral, but, in our opinion, 

 not so. '^ The Hnes," says Dr. Wright, " are seen to consist of the remains of 

 the tmmpet-shaped mouth, which is partially absorbed as the tube increases 

 its length, but stiU remains as a slightly-overlapping ridge over the new part 

 of the tube growing within it. The groove thus formed is filled up with the 

 * coUetoderm.' The spiral character seems to be in some way connected with 

 this mode of growth ; but I have not satisfied myself in what way." In a 

 subsequent letter he writes — ^' The chitinous matter of each successive ring 

 is not continuous with that of the lings above and below it ; it is only at- 

 tached to it by the inner lining of sarcode and by its outer covering (XXXI. 

 12, 13). We have by this condition a provision for the growth of the tube, 

 both in width, length, and thickness, similar to that which occurs in the 

 shell of Echinus, Growth in length may be eff'ected by deposition of chitine 

 on the upper and lower edge of each ring, growth in breadth by the gradual 

 unrolling of the spiral, while a continuous deposition of hard matter from 

 the inner lining of sarcode thickens and strengthens the whole tube." 



