WORMS 



187 



These photoreceptors are of the most varied types and many species 

 possess eyes of more than one variety. The neuro -sensory cells may be 

 either apolar in type provided with an internal optic organelle, or 

 bipolar, provided with a ciliated or striated border ^ : they may occur 

 as single cells or in groups forming an eye of either the subepithelial or 

 epithelial variety, in which case it may show a flat, cupulate or vesicular 

 arrangement. Pigment is a constant association, situated within the 

 sensory cells or in special supporting cells. If a refractive medium is 

 present it may be formed either from the retinal or the epidermal cells, 

 while light-refracting structures are usually cuticular in origin. As a 

 general rule their function can only be the primitive ability to detect 

 light, but the visual organs of some types, such as some polychaete 

 worms, are structurally capable of some degree of localization and 

 resolution (a directional eye) and perhaps even of visual imagery. 



UNSEGMEXTED WORMS 



The unsegmented worms may be divided into three phyla — flat- 

 worms, ribbon-worms, and thread-worms. 



1. PLATYHELMINTHES or FLAT-WORMS Constitute a gi(jup of very simply 

 organized creatures the members of which show the progressive degeneration 

 associated with parasitism. It is divided into 3 main classes : 



(a) TURBELLARiANS, freely -Swimming leaf-shaped aquatic creatures of 

 carniv^orous habit, frequenting brackish or salt water or moist places on land ; 

 the name is derived from the turbulence caused in the water by the beating of 

 their cilia when they swim. They are classified accoi'ding to the arrangement 

 of the gut — the minute marine Actf-la (without intestine), the small salt and 

 fresh-water Rhabdocojla (rod-shaped intestine), the (mainly) marine Alloeocoela 

 (irregular intestine), the small, flat, elongated Tricladida (3-branched intestine) 

 found in fresh or salt water or on land (including the Planaria), and the large, 

 leaf-like, marine Polycladida (many-branched intestine). 



(6) TREMATODES or FLUKES, leaf-like parasites, external or internal, found 

 on or in all types of \'ertebrates, clinging to their hosts with suckers. Examples 

 are the liver-fluke, Fasciola hepatica, which infests the livers of sheep and 

 cattle, or the Schistosoma Juematobia which causes bilharziasis. 



(c) CESTODES or TAPE-WORMS, endoparasites, frequenting the alimentary 

 canal of Vertebrates, including domestic animals and man, such as Taenia eckino- 

 coccus, or T. solium. 



2. NEMERTiNES or RIBBON-WORMS, ribbon- or thread-like in shape, often 

 vividly multi-coloured, varying in size from under an inch to enormous lengths 

 (25 metres in Linens) and provided with cilia and a remarkable retractile pro- 

 boscis forming a tactile organ used to capture prey. Most are marine in habitat, 

 creeping in the mud and under stones ; a few are found in fresh-water (Prostoma); 

 some are terrestrial (Geonemertes) ; and a few live commensally with bivalves 

 or ascidians. 



3. NEMATODES, ROUND- Or THREAD-WORMS, Cylindrical in shape and often 

 minute, which teem in the soil or in water and are often endojjarasitic in plants 

 and animals (Ascaris, Trichinella, Ankylostoma, Filaria, etc.) ; but free-living 

 forms occur at any rate in part of the life-cycle. 



1 p. 127. 



Polyclad, 

 Leptoplana 



Schistosoma 



Teen in 

 echinococcus 



