176 THE INVERTEBRATA 



The neuromuscular system is further developed than in even the 

 medusoid individuals of the Hydrozoa. The muscles are ectodermal, 

 and each cell is almost entirely converted into contractile protoplasm 

 vi^ith a cross-striated pattern forming an elongated fibre; physio- 

 logically they are capable of rapid rhythmic contraction and not of slow- 

 tonic contraction like the muscle of a sea anemone (p. 193). The fibres 



p,cn. 



I. en. 



a,cn 



p.en. 



x.cn. 



Fig. 132. 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.cn. interradial canal; o.o.a. opening on oral arm. Other 

 letters as in Fig. 131. 



are arranged as a circular musculature over the peripheral part of the 

 subumbrella. The nerve net is also confined to the ectoderm and is 

 concentrated in the neighbourhood of the tentaculocysts. There is 

 no true velum, but a pseudovelum consisting of an internal flange 

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

 The tentaculocysts are the characteristic sense organs of the 



