MICROSCOPIC FORMS OF ANIMAL LIFE. 43 



Protophytes. It is sometimes hardened, so as to form a * shield ' that 

 protects the body on one side only, or a 'krnca' that completely invests 

 it ; and there are other cases in which it is so prolonged and doubled 

 upon itself, as to form a sheath resembling the ( cell ' of a Zoophyte, 

 within which the body of the Animalcule lies loosely, being attached 

 only by a stalk at the bottom of the case, and being able either to project 

 itself from the outlet or to retract itself into the interior. In a curious 

 group lately described by Haeckel, consisting of Infusoria that spend 

 their lives in the open sea, the body is inclosed in a siliceous lattice-work 

 shell, usually bell-shaped or helmet-shaped, which bears so strong a 

 resemblance to the shells of many Eadiolaria as to be easily mistaken 

 for them. The form of the body is usually much more definite than 

 that of the naked Rhizopods ; each species having its characteristic 

 shape, which is only departed from, for the most part, when the Animal- 

 cule is subjected to pressure from without, or when its cavity has been 

 distended by the ingestion of any substance above the ordinary size. 

 The cilia and other mobile appendages of the body are extensions of the 

 outer layer of the ' ectosarc ' proper ; and this layer, which retains a high 

 degree of vital activity, is sometimes designated the 'cilia-layer.' Be- 

 neath this is a layer in which (or in certain bands of which) regular, 

 parallel, fine striae may be distinguished ; and as this striation is also 

 distinguishable in the eminently contractile footstalk of Vorticella (Fig. 

 305, B), there seems good reason to regard it as indicating a special 

 modification of protoplasmic substance, which resembles muscle in its 

 endowments. Hence this is termed the ' myophan-layer.' Beneath 

 this, in certain species of Infusoria, there is found a thin stratum of 

 condensed protoplasm, including minute 'trichocysts/ which resemble 

 in miniature the 'thread-cells' of Zoophytes ( 528); and this, where it 

 exists, is known as the ( trichocyst-layer.' 



437. The vibration of ciliary filaments, which are either disposed 

 along the entire margin of the body, as well as around the oral aperture, 

 (Fig. 305, A, B), or are limited to some one part of it, which is always in 

 the immediate vicinity of the mouth (Fig. 304), supplies the means in 

 this group of Infusoria, both for progression through the water, and for 

 drawing alimentary particles into the interior of their bodies. In some, 

 their vibration is constant, whilst in others it is only occasional. The 

 modes of movement which Infusory Animalcules execute by means of 

 these instruments, are extremely varied and remarkable. Some propel 

 themselves directly forwards, with a velocity which appears, when thus 

 highly magnified, like that of an arrow, so that the eye can scarcely fol- 

 low them ; whilst others drag their bodies slowly along like a leech. 

 Some attach themselves by one of their long filaments to a fixed point, 

 and revolve around it with great rapidity; whilst others move by undu- 

 lations, leaps, or successive gyrations : in short, there is scarcely any 

 kind of animal movement which they do not exhibit. But there are 

 cases in which the locomotive filaments have a bristle-like firmness, and, 

 instead of keeping themselves in rapid vibration, are moved (like the 

 spines of Echini) by the contraction of the integument from which they 

 arise, in such a manner that the Animalcule crawls by their means over 

 a solid surface, as we see especially in Trichoda lynceus (Fig. 308, p, Q). 

 In Chilodon and Nassula, again, the mouth is provided with a circlet 

 of plications or folds, looking like bristles, which, when imperfectly 

 seen, received the designation of ' teeth ;' their function, however, 

 is rather that of laying hold of alimentary particles by their expansion 



