108 



COELENTERATA. 



absence of the oesopliageal tube* and mesenteries, but it recalls the 

 Anthozoan-polyp in the fact that the structureless lamella is developed 

 into a gelatinous layer, and in the presence of gastral ridges (taenioles) 

 or folds of endoderm resembling rudimentary mesenteries, into which 

 the jelly is continued. It is somewhat cup-shaped (Fig. 92), and 

 is attached by the aboral end which is elongated and narrow, and often 

 secretes a chitinous tube for fixation. There are eight or sixteen 

 tentacles (Fig. 93) round the^mouth supported by a central axis 

 of stiff endoderm cells. There are four gastral ridges or taenioles 

 (Fig. 94), each of which is accompanied by a longitudinal muscle 

 derived from the endoderm. 



The Medusa is a free-swimming animal, and consists of a flattened 



disc or arched 

 bell of gelatinous 

 consistence, from 

 the under or sub- 

 umbrella surface 

 of which hangs 

 a central stalk 

 the luanulrium 

 bearing at its free 

 end the mouth. 

 The greater part 

 of the umbrella 

 consists of the 

 jelly or enlarged 

 structureless la- 

 mella ; this is 

 often traversed 

 by protoplasmic 

 strands which 



may contain nuclei. In the normal position the medusa swims 

 by the contraction of the bell with its convex or ex-umbrella surface 

 upwards. The manubrium is frequently prolonged in the region of 

 the mouth into lobes and tentacle-like structures, while the edge of 

 the umbrella is beset with a variable number of true tentacles, 



In a few cases the medusae are attached by the ex-umbrella surface ; in the 

 Lucernaridae (Fig. 134) by a styliform prolongation of the aboral pole ; in some 

 Rhizostomas by a sucker-like plate of the ex-umbrella. Some of them can creep 



m.L- 



FIG. 94. Transverse section through the middle part of a Scy- 

 phistoma. ek ectoderm ; en endoderm ; s jelly (structureless 

 lamella) ; ml longitudinal muscle ; gw gastral ridge; gv enteron 

 (from Chun, after Claus). 



* Gotte asserts that there is an oesopliageal depression of ectoderm in the 

 Scyphopolyp. 



