188 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 (Cyclostomata) 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 
into 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 
Si are able to attach themselves to other 
Baas ach of enorey fishes, the flesh of which they rasp away, 
using the tongue for the purpose, this 
being moved by means of powerful muscles. 
MOLLUSCS (Mottusca) AS PARASITES 
Certain Sea-Snails afford the best illustrations of the parasitic 
habit as occurring among Molluscs. One of the Cap-Shells (Zhyca 
ectoconcha, fig. 1134) is an external parasite upon a kind of Star- 
Fish (Lznckia mutltiforis). 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- 
