198 LIFE; IN ITS INTERMEDIATE FORMS. 
and row it swiftly along in any given direction at tlie 
creature’s will. 
The skin in this species is membranous, but somewhat 
firm, folded upon itself at certain points of the circum- 
ference, so that one portion can, to a certain extent, be 
sheathed within another, as in the Annelida and in the 
larval of Insects. In many genera, however, the integu- 
ment hardens into a firm horny shell (lorica), which takes 
different forms, the most common being that of a tortoise- 
like box, as in Brachionus, &c., with a wide orifice in fiont 
(often armed with projecting spines) for the protrusion 
and expansion of the ciliary wheels, and a smaller one m 
the rear to give issue to the flexible-jointed foot, which 
commonly terminates in two pointed toes. The structure 
and composition of this integument are similar to those 
of Insects and Crustacea. 
Owing to the crystal transparency of these tiny crea- 
tures, we are enabled, with the aid of microscopic powers, 
to discern their internal structure and economy with a 
clearness which no dissection would avail to shew ; even 
were not dissection precluded by their minuteness. They 
are thus perceived to possess a comparatively high condi- 
tion of organisation. The muscular, the nervous, the 
digestive, and the reproductive systems are, in particular, 
well developed. 
Lon o- and broad ribbons of muscular texture pass from 
organ to organ, playing freely in the general cavity of the 
body, their contractions and extensions plainly visible. 
These are very numerous, some passing down longitudinally, 
others transversely, their extremities being inserted into 
the lining tunic of the integument. Sometimes the muscles 
