INVERTEBRATE ZOOLOGY. 81 



examination of a tropical rotifer. Trochnsphaera , which appears to be a modi- 

 fied Trochozot'in, the group has gained in interest, as in it we have probably 

 lineal descendants of that ancient form, differentiated and modified through 

 adaptation to special environments. 



54. Rotifers are best studied alive in a drop of water, as in the case of 

 Infusoria. Search in different places in an aqiiarium. particularly in the slime 

 on the under side of leaves and among the material at the bottom. They are 

 distinguished by an elongated tail, composed generally of joints, and by the 

 characteristic " wheel apparatus." which they unfold at the anterior end, and 

 often suddenly i-etract. Obtain a good specimen, jareferably one whose mo- 

 tion is circumscribed by debris, and observe the following points :— (a) The 

 three body regions : retractile portion or head, main body region or thorax. 

 and tail or foot, (b) The '• wheels :" these are in the typical forms a pair of 

 lobes covered with cilia which move in such a way as to give to the lobes the 

 appearance of revolving wlieels. These are probably modifications of the 

 prae-oral band or trochiis of the ancestral form, (c) Notice the stream of 

 particles drawn into the bodj^ between the wheels : follow it inwards and 

 notice the masticatory apparatus, a set of chitinous teeth continually mo\dng 

 back and forth. The other organs of the bodj' are difficult to distinguish and 

 may be studied by the help of text books [cf. Vogt u. Jung. pp. 424-44;")]. 

 The sexes are separate, but the males are very small and comparatively rare, 

 appearing only at certain times in the year. The females lay two sorts of 

 eggs. The first or svninier eggs are parthenogenetic. soft shelled, and of two 

 sizes, the larger one producing females and the smaller ones males. The sec- 

 ond sort, the winter eggs, are the product of copulation : they have hard 

 shells generally marked with curious and complex sculptures, and produce 

 females alone. In some species {Brach ion >is, etc..) the eggs, when laid, remain 

 (dinging to the body of the parent, while in others they are deposited in rows 

 on water plants. 



55. (rastrotricha. These are a small grouj) of animals, allied to Rotifers 

 and occurring with them. Thej' have a fish-like form, but are depressed and 

 have a broad ciliated douljle stripe of cilia along the ventral surface, making 

 them appear like hypotrochous Infusoria. They may be considered as modifi- 

 cations of the Trochozocin. but in a different direction from that taken l)y the 

 Rotifers. In these the trochus has disappeared, the ventral stripe alone per- 

 sisting as the locomotive organ. 



