loo 



ZOOLOGY 



SECT. 



Lying parallel with the inner or concave border of each gonad 

 is a row of delicate filaments (g. /), formed of endoderm with a 

 core of mesogloaa and abundantly supplied with stinging-capsules. 

 These are the gastric filaments ; their function is to kill or 

 paralyse the prey taken alive into the stomach. No such endo- 

 dermal tentacles are known in the Hydrozoa. 



Muscular and Nervous Systems. The contractions of the 

 bell by which the animal is propelled through the water are 

 effected by means of a muscular zone round the edge of the sub- 

 umbrella. The nervous system is formed on a different plan 

 from that of the hydroid medusae. Instead of a double nerve-ring 

 round the margin of the umbrella, there are eight groups of nerve- 

 cells in connection with the marginal notches. The nerve-cells 

 lie between the bases of the epithelial cells, and external to the 

 muscular layer : they are obviously ectodermal structures. 



The sense organs are lodged in the marginal notches in close 

 relation with the nerve-patches : like the latter, therefore, four of 



B 



oe 



FIG. 115. Aurelia aurita. A, small portion of edge of umbrella, showing the relations of the 

 tentaculocyst ; B, vertical section of the same region (diagrammatic) ; h, hood ; I, lithite ; 

 ///:/. I)*, marginal lappet ; oc, ocellus ; olf. 1, olf. 2, olfactory pits. (Altered from Lankester.) 



them are per-radial and four inter-radial. Each consists of a 

 peculiar form of sense-club or tentaculocyst, containing a prolonga- 

 tion of the circular canal, and thus representing a hollow instead 

 of a solid tentacle. At the extremity are calcareous concretions 

 or lithites (/) derived from the endoderm, and on the outer side 

 is an ectodermal pigment-spot or ocellus (oc). The tentaculocysts 

 are largely hidden by the marginal lappets (nig. Ip) and by a 

 hood-like process (k) connecting them ; and in connection with 

 each are two depressions, one on the ex-umbrella (olf. 1), the other 

 immediately internal to the sense-club (olf. 2) : these depressions 

 are lined with sensory epithelium and are called olfactory pits. 



The development and life-history of Aurelia present several 

 striking and characteristic features. The impregnated egg-cell 



