154 



THE INVERTEBRATA 



protoplasm with a cross-striated pattern forming an elongated 

 fibre ; physiologically they are capable of rapid rhythmic contraction 

 and not of slow tonic contraction like the muscle of a sea anemone. 

 The fibres are arranged as a circular musculature over the peripheral 

 part of the subumbrella. The nerve net is also confined to the ecto- 

 derm and is concentrated in the neighbourhood of the tentaculocysts. 

 There is no true velum, but a pseudovelum consisting of an internal 



jy.cn- 



a.cn. 



p,cn. 



t.cn. 



Fig. 129. Diagram showing the course of ciliary circulation (see arrows) 

 in the genital pits and other organs of an adult Aurelia. After Widmark. 

 A, interradius ; B, perradius ; gen. gonad; gg.cn. gastrogenital canal; gst.p. 

 gastric pouch; i.ai. interradial canal; o.o.a. opening on oral arm. Other 

 letters as in Fig. 128. 



flange which is not occupied by muscles and a nerve ring as in the 

 Hydrozoa. 



The tentaculocysts are the characteristic sense organs of the 

 Scyphomedusae (but are present also in the Trachomedusae and 

 Narcomedusae of the Hydrozoa). They are minute tentacles which 

 project at the end of the interradial and perradial canals, which are 



