282 Characteristics of Rotifers. 



in many cases enjoy the further protection of a sort of armour, 

 which may be roughly compared to that of the turtle or tor- 

 toise, or of dwelling-tubes, more or less simple in construction. 

 They have considerable powers of contracting or expanding 

 their bodies, and in some cases can slide one portion inside 

 another, like the tubes of a hand telescope. Their possession 

 of a certain rank in the animal world is evidenced by a well- 

 marked digestive canal, consisting of an oral or receiving 

 orifice, an apparatus composed of hard materials for crushing and 

 grinding food, popularly termed a " gizzard," though it is not 

 exactly analogous to the grinding organ of birds, a gullet, a 

 stomach supplied with secretions from glandular organs, and, 

 except in one genus (Asplanchna) , an intestine and anal orifice. 

 The term " oral orifice " has been used instead of the " mouth," 

 to avoid suggesting comparisons which might not be correct 

 between the mouth of the rotifer and that of the higher animals 

 of the vertebrate type. More or less surrounding the oral 

 or receiving orifice, are groups of cilia, which in the common 

 rotifer present the aspect of the well-known " wheels." Most 

 rotifers, if not all, possess rudimentary eyes, either at an early 

 period, or throughout their lives. They have all a distinct 

 muscular system, and a nervous system, of which a large cere- 

 bral ganglion is frequently conspicuous. None of them possess 

 a true blood circulation, but all have what is termed a " water- 

 vascular system " of delicate canals, similar to those found in 

 Turbellaria, to which the Planaria, well-known to microscopists 

 belong, and in the Helminths, or intestinal worms — flukes, 

 tapeworms, and threadworms. The functions of the water- 

 vascular system are not positively known, but are conjectured 

 to be respiratory and excretory. The contractile organ, 

 popularly called the " heart " of rotifers, belongs to this 

 system, and so do the " vibratile tags " easily seen in some 

 species. 



The rotifers were formerly considered to be hermaphrodites, 

 but Mr. Brightwell and Mr. Gosse discovered distinct males of 

 certain species, and it seems most probable that other males 

 will be found in the course of further research. The rotifers 

 usually obtained by microscopists are females, and their eggs 

 and ovaries are frequently very conspicuous, the eggs often 

 being enormously large in proportion to the dimensions of 

 their parents. The common rotifer is an example of ovoviva- 

 parous generation, the egg with its living contents being 

 commonly seen in the body of the parent, and the young coming 

 forth as the exact image of its mother. In the common rotii 

 and in many others when the eggs are sufficiently advanced, 

 the jaws of tlic unborn infanl may be situ vigorously working, 

 the little redeyes brightly shining, ami a strong ciliary current 





