PARAZOA 



181 



{Paramoecmm) sensitivity to light is diffuse ; in other Cihates {Stentor) 

 it is localized to a part of the organism but without apparent specific 

 mechanism ; but even at this primitive unicellular stage an obvious 

 localization of function may be attained by the development of an 

 EYE-SPOT and the efficiency of the organelle increased, particularly in 

 the acquirement of a crude directional appreciation, by the provision 

 of pigment (as in Euglena) ^ or even of a primitive refractile mechanism 

 (as in some Dinoflagellates).^ 



Parazoa 



The SPONGES (porifera), sessile marine animals which form living 

 thickets in the sea, represent a cul-de-sac in evolution between Protozoa 

 and Metazoa dating back almost to the beginning of geological records. 

 They are the simplest multicellular animals and show the beginnings 

 of the development of a "" body " composed of tissues ; but although 

 there is cellular differentiation there is little cellular co-ordination. 

 Being vegetative and sedentary in habit they have no need of sense- 

 organs as they lie moored to rocks or sea-weed. They possess no nerve 

 cells but the body cells retain properties of an irritability of a low 

 level ; and in the active larval forms of certain types (the simple 

 sponge, Leucosolenia) apolar light-sensitive cells of the most elementary 

 type have been described (Minchin, 1896). 



Invertebrate Metazoa 



In Metazoa — which includes all animal species apart from the 

 Protozoa and Parazoa — -the development of specialized cells and their 

 eventual co-ordination into distinct organs allow the evolution of 

 specific sensory activities as the term is generally understood. These 

 we shall now study, but it must be remembered that the Invertebrates 

 (or Non-chordates) do not form a homogeneous sub-kingdom but rather 

 represent an assemblage of unrelated groups of animals which have 

 little in common except the negative attribute of not being provided 

 with a dorsal nerve-cord with its supporting axis or with gill-slits. 

 From our restricted point of view there is the dramatic diff"erence that 

 (with few exceptions) the eye when present is developed from the skin, 

 while in Vertebrates it originates as an outgrowth of the brain. 



cgelexterata 



CCELENTERATES are simply formed animals with a body-cavity 

 (coelom) and digestive cavity(enteron) combined so that the body is formed 

 as a sac with an opening at one end only. They show the beginnings of 

 separate organs witli a consequent division of labour, and among them 



Leucosolenia 



Sycon 



126. 



- p. 126. 



