198 



LIFE; IN ITS INTERMEDIATE FORMS. 



and row it swiftly along in any given direction at the 

 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 

 larvae 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, etc., with a wide orifice in front 

 (often armed with projecting spines) for the protrusion 

 and expansion of the ciliary wheels, and a smaller one in 

 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. 



Long 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 



