182 



THE EYE IN EVOLUTION 



Hydra 



Obelia medusoid 



Obelia polyp 



Sea-anemone 



Comb jelly 

 (see Figs. 887-8) 



visual structures of some complexity first make their appearance. The 

 phylum may be divided into two sub-phyla — the cnidaria, provided 

 with numerous stinging cells (kvlSy), a nettle), and acnidaria, wherein 

 these are replaced by adhesive cells. The first-sub-phylum is divided 

 into 3 classes : 



HYDROZOA, comprising solitary polyps such as tlie fresh-water Hydra, the 

 marine Hydroids, branching colonial polyps of vegetative appearance liberating 

 freely-swimming Hydromedusie {Obelia, Sarsia, etc.) and some j^elagic colonial 

 forms. 



SCYPHOZOA (" cup animals "), marine jellyfish, free-swimming medusae, 

 typically umbrella-shaped with the important organs situated on the margin or 

 under-surface. 



ANTHOZOA (" flower animals "), sessile marine polyjas with no medusa- 

 forms, such as sea-anemones, sea-fans, sea-pens and corals. 



ACNIDARIA, comprising the Ctenophnra (comb-jellies or sea-gooseberries), 

 delicate freely-swimming globular organisms, pelagic in habit, gelatinous and 

 transparent, beautifully iridescent in the sunlight and often luminescent in the 

 dark,^ provided with comb-like rows of cilia. 



The degree of elaboration of the visual receptors varies with the 

 motility of the organism, and many Coelenterates are sessile, plant-like 

 zoophytes ; eyes are therefore confined to the mobile medusae and these 

 are of a very primitive nature,^ while the sessile polyps of this phylum 

 (hydroid forms and all Anthozoa) have no sense organs or, at most, 

 contact photoreceptors of the most elementary type.^ 



The Ctenophora are provided with a sense organ at the upper pole of the 

 organism consisting of a mass of limestone particles sup- 

 ported on cilia associated with sensory cells communicating 

 by nerve fibrils with the swimming-combs; this is considered 

 to act as a statocyst or balancing device and visual organs 

 are absent. 



Among the Hydrozoa, some fresh-water forms 

 are sensitive to light but possess no detectable visual 

 organs ; a hydra, for example, will migrate towards 

 the lighted side of its container where, incidentally, 

 there are usually more food-organisms. In some 

 freely-swimming Hydromedusae, however, externally 

 visible light-sensitive organs provided with sensory 

 cells and pigment and sometimes a refringent appara- 

 tus may be found in the tentacular bulbs at the bases 

 of the tentacles (Fig. 162) ; these take the form of 



' p. 739. 

 ^'ir detailed information, see O. and R. Hertwig (1877) 

 Ber^ S98), Linko (1900), v. Uexkull (1909), Lehmann (1923). 



il6. 



Fig. 162.— The 

 Medusoid Form 

 OF BoviiAi.sriL- 

 LEA (Margel/.s). 

 M, manubrium; 



R, radial canal ; 



S, sense organ 



(after Allman). 



Schewiakoff (1889), 



