204 



COELEXTERATA. 



hooks for the retention of food. The hepatic bands are absent. 



The edges of the polar plates of the sense-organ are dendritically 



branched. The interradial vessels and the 

 ampullae spring directly from the funnel. 

 The musculature is well developed, and the 

 meridional and paragastric vessels give nu- 

 merous branches which anastomose. 



Thread-cells are almost universally absent ; 

 but the ectoderm of the tentacles contains 

 peculiar " adhesive cells," the base of which 

 is prolonged into a spirally coiled thread 

 (Fig. 161), while the convex free end is soft 

 and glutinous and becomes readily attached 

 to any object which touches it. Thread-cells 

 are found in the ectoderm of Eucldora alone, 

 the tentacles of which are characterized by 

 the absence of adhesive cells and the presence 

 filaments of the tentacle of O f amoeboid ectodermal prominences. 



Euplocarnis stationis (after 



The nervous system and sense-organs have 

 already been described (p. 199). 



The Ctenopliora are hermaphrodite, and 

 sexual reproduction alone is known. The generative cells arise on 

 the walls of the meridional vessel, or of diverticula of the same. 

 Sometimes they are localized 

 (Cestiis) ; sometimes they arise 

 along the whole length of the 

 canals, one side being beset by 

 egg-follicles, the other by sperm- 

 sacs. Ova and spermatozoa 

 when ripe pass into the gastro- 

 vascular space, and are ejected 

 through the apertures of the 

 same. 



FIG. 161. Muscle-fibres, ad- 

 hesive cells (A/) and tactile 

 cells (I) from the lateral 



R. Hertwig). kf prolon- 

 gation of the contractile 

 thread of a prehensile cell. 



The early development takes place 

 within the egg -membranes. The 

 segmentation is complete. A cap of 

 small ectoderm cells is soon formed 

 (Fig. 163), which grow round the 

 larger endoderm cells ; but before 

 this overgrowth is completed a number of cells are separated from the lower 

 ends of the large endoderm cells, and constitute an embryonic mesoderm (Fig. 

 163, 5, Ms] (Metschnikoff). The mesoderm gives rise to the protoplasmic 



FIG. 102. Meridional vessel (Gc) of Beroe ovata 

 with ova (Ov) and spermatozoa (Sj>) in their 

 walls (after Will). 



