i88 ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



inadvertence or any other cause, adopted the habit of casually 

 dropping her eggs in a neighbour's nest, might thereby ensure 

 a profitable inheritance for endless generations of her offspring. 

 This much granted, all the rest will follow easily enough, but 

 it must be confessed that this is only a presumption, though a 

 presumption which seems plausible if not likely." 



FISHES (PISCES) AS PARASITES 



- 



The Lampreys and Hags (Cyclos tomato) are scaleless, eel- 

 shaped creatures, devoid of jaws, and in- 

 termediate in habit between carnivorous 

 forms and external parasites. On the 

 under side of the head is a bell-shaped 

 sucker, the lining of which is thickened 

 TONGUE m t o a varying number of sharp horny 

 teeth. At the top of the bell is the true 

 mouth-opening, provided with a projecting 

 " tongue", also tooth - bearing (fig. 1133). 

 By means of the sucker these creatures 

 are able to attach themselves to other 

 fishes, the flesh of which they rasp away, 

 using the tongue for the purpose, this 

 being moved by means of powerful muscles. 



Fig. 1133. Sucker of a Lamprey 

 {Petromyzon} 



MOLLUSCS (MOLLUSCA) AS PARASITES 



Certain Sea- Snails afford the best illustrations of the parasitic 

 habit as occurring among Molluscs. One of the Cap-Shells (Thyca 

 ectoconcha, fig. 1134) is an external parasite upon a kind of Star- 

 Fish (Linckia multiforis). It will be seen from the illustration 

 that this form is still easily recognizable as a mollusc, though the 

 influence of its particular mode of life is also obvious. The mouth 

 has shifted backwards, and is on the end of a short proboscis, 

 which penetrates into the body of the host, and is surrounded 

 by an adhesive disc, formed by the fusion of parts of the foot 

 with an outgrowth from the head. The characteristic rasping- 

 organ (odontophore) has entirely disappeared, and the pharynx 

 has been converted into a sort of suction-pump by which the 

 juices of the star-fish are drawn in. The body of the same un- 



