204 



COELENTERATA. 



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 Euclilora alone, 

 the tentacles of which are characterized by 

 the absence of adhesive cells and the presence 

 of amoeboid ectodermal prominences. 



The nervous system and sense-organs have 

 already been described (p. 199). 



The Ctenoplwra 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 



(Cestus) ; 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 (fc/) and tactile 

 cells (6) from the lateral 

 filaments of the tentacle of 

 Euplocamis stationis (after 

 R. Hertwig). ltf> 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. 162. Meridional vessel (Gc) of Beroe ovata 

 with ova (Ov) and spermatozoa (Sp) in their 

 walls (after Will). 



