136 CCELENTERATA OR STINGING-ANIMALS. 



DIAGRAM VI. 



The Structure of Ccelenterata. 



In the centre of the page is a typical gastrula (Ca.) 5 on the lower 

 half of the pj^e are Hydrozoa, on the upper half are Scyphozoa. 



At the lower left-hand corner is a simple diagram of Hydra {Ify.), 

 showing two tentacles, the mouth, a daughter bud, a testis, and the 

 ovary. An ideal cross-section is represented by Sec, while some of the 

 characteristics of the ectoderm and endoderm are suggested above. At 

 the right-hand corner are two stinging-cells {St.), one with the lasso 

 ejected, the other with it still coiled in the cavity of the cell. 



The diagram marked ZoopA. suggests the origin of a medusoid bud 

 (mal.) from a hydroid colony, while on the opposite side of the page 

 the nature of the velum, the darkened endodermic lining of the digestive 

 cavity, and the hoUowness of the tentacles are represented in two figures 

 of medusoids (VI/ei2'.). Above the line, at Au. is a section of .^«r(;/M, 

 showing the gastric filaments (^.f.) projecting into the digestive cavity, 

 and the sub-genital pockets {g: p.) (after Ray Lankester). At H. T. is 

 represented a Hydra-tuba with a basal stolon ; the two figures marked 

 Sir. show Strobila-stages ; Eph. is a liberated Ephyra. 



At Anem. is a vertical section of a sea-anemone (after Hertwig). 

 Two mesenteries are exposed ; each is perforated by two pores ; each 

 bears muscles, and digestive filaments (/!), and reproductive organs (?-.). 

 The central cavity is bounded above by an inturned gullet ; two tentacles 

 (t. ) are also shown. To the right are two cross sections, of a Zoantharian 

 (Z.) and of an Alcyonarian {AL), showing how the muscles (represented 

 by black marks) lie on the mesenteries 



