74 ZOOLOGY FOR MEDICAL STUDENTS chap. 



and the mouth occupied by a sHghtly convex disc, surrounded by a 

 groove deepened at one point into a funnel-shaped recess (the vestibule) 

 which is continued down into the gullet and into which opens the anus. 

 Disc, giroove and vestibule make up the peristome, and the row of cilia 

 runs round the disc and down into the vestibule round which it twists 

 in the form of a continuous undulating membrane like that in the gullet 

 of Paramecium. The macronucleus of Vorticella (Fig. 29, B, N) is 

 elongated and is usually bent into a more or less horse-shoe shape, and 

 there is a single small micronucleus (w). A contractile vacuole is present 

 {c.v) which empties itself into a little pocket-like recess in the wall of 

 the vestibule — the reservoir. 



The Vorticella is extremely sensitive and at the slightest shock the 

 body contracts to a spherical shape the disc being drawn down and the 

 outer lip of the surrounding groove contracting over it while the stalk 

 becomes coiled into a close spiral. As in the case of Stentor contraction 

 is brought about through the agency of highly developed myonemes. 

 In the stalk when extended a single myonemic band may be seen 

 running throughout in a spiral course ; when this contracts it tends to 

 straighten and the stalk containing it assumes on the contrary a spiral 

 twist. 



Another member of the Peritricha should be mentioned as it may be 

 observed by the student during his work on Hydra. This is Trichodina, 

 a curious disc-shaped ciliate which occasionally may be seen gliding 

 about on the surface of a Hydra. Like Kerona it is found only as a 

 parasite of Hydra and other aquatic animals. 



ACINETARIA 



As an appendix to the Ciliata may be mentioned the group Acinetaeia 

 or Suctoria, comprising creatures which have given up the actively 

 moving habit and have undergone characteristic modifications in corre- 

 lation with their sedentary mode of life. 



The typical Acinetarian (Fig. 30, A) consists of a mass of protoplasm, 

 commonly pear-shaped or in the form of a somewhat triangular disc, 

 attached to the solid substratum by the narrow end which is more or 

 less prolonged to form a stalk. The complex structure of the ectoplasm 

 seen in the typical actively moving ciliate has disappeared. At the 

 attached end the ectoplasm is connected with the substratum by secreted 

 material — which may be looked on as ectoplasm that has lost its organized 

 structure — and it is this which may take the form of a long stalk. In 

 some cases the material of the stalk is continued upwards so as to 



