VIII. 



Rotifera, Gastrotricha and Kinorhyncha 



Rotifers. The usual type of nervous system of the female is 

 a dorsal ganglion or brain from which slender nerves pass to ten- 

 tacles, the ciliary disc or the general body. In Disco-pus, Zelinka 

 shows a ventral oesophageal ganglion as well as the usual dorsal 

 one. The shape of the brain or dorsal ganglion differs somewhat 

 in various species being almost spherical in some and quite elongate 

 in others, and in many cases bi-lobed. In Frullaria several longi- 

 tudinal strands of the nervous system have been shown. At nodal 

 points ganglion cells are located. The peripheral nerves are chiefly 

 as follows : one to each of the tentacles ; a pair of lateral nerves 

 which descend into the body and divide into two main branches, 

 one more ventral and one lateral, which give off numerous lateral 

 divisions to the muscles and viscera. Many fine branches run from 

 the brain to the ciliary ring to end in intimate relation with the 

 ciliary cells. 



Antennae or feelers, usually three in number, a median dorsal 

 and two lateral ones are supplied by definite nerves. Each of these 

 structures consists of a small cluster of sense hairs born on a 

 slight swelling which receives the nerve. Sometimes the antennae 

 are retractile by means of internal muscle. These antennae may be 

 organs for touch or smell or both. 



The brains of some forms contain a sac full of mineral mate- 

 rial. This may be some sort of sense organ, possibly a statocyst. 



The eye-spot in its simplest form is a refractive globule in a 

 red pigmented cup to which latter nerve fibers pass, or the eye or 

 eyes may rest directly on the brain. Sometimes two eyes occur and 

 these may be very close together, almost like one. In some species 

 the eyes are just under the ciliary band or within the disc. A me- 

 dian and two lateral eyes occur in some forms, or even another pair 

 of eyes may also be found. In some, pigment spots occur at the 

 hind end of the body. 



It has been suggested that some rotifers are able to avoid ob- 

 jects by means of a sense of sight aided by the tactile and olfactory 

 sense. 



The chief work on the nervous system of this group has been 

 by Zelinka, 1888-90, Cast, 1900, and Halva, 1905. The more recent 

 work of Hirschfelder, 1910, has added quite a little to our knowl- 

 edge of the nervous system and sense organs in a number of forms. 

 This last author recognizes four general types of nerve cells which 

 grade into each other to some degree. Nerve fibers are described 

 as containing a central core of fibrils, an intermediate covering and 

 an outer sheath. Cells ai-e uni- or bi-polar; the last kind has one 



