34 THE MOLLUSCA 
Stylifer (Fig. 20); or, like Hulima and Entovalva, are endoparasitic, 
also in Echinoderms. In the latter case the animal may become so 
degenerate in the adult state as to consist of little more than a 
sac containing the genital products, as for example Entosiphon 
(Fig. 21), Entocolax (Fig. 138), Entoconcha (Fig. 139), and Enteroxenos 
(Fig. 140). 
There are many cases of protective adaptation and mimicry in 
the various groups of Mollusca. The pelagic species are generally 
transparent and colourless or 
tinged with blue, but the most 
remarkable examples of mimicry 
are found among the forms un- 
protected by a shell (Nudibranchs, 3 
Lamellaria, etc.), in which the a 
ow 
Fic. 20. Fic. 21. 
Stylifer celebensis, X 12. pr, pro- Entosiphon deimatis, x 2. 0, orifice 
boscis ; ps, pseudopallium ; sp, spire of the proboscis; ov, ova; pr, pro- 
of the shell not covered by the pseudo- boscis ; ps, pseudopallium ; s, fixative 
pallium. (After Kiikenthal.) siphon, (After Koehler and Vaney.) 
individual assumes the colour and aspect of its habitat, as for 
instance Hermaea dendritica on green algae, and Hermaea bifida on 
Griffithsia, a red alga, etc. The abyssal Molluscs are colourless, 
and are characterised by the thinness of the shell, the atrophy of 
the visual organs, and the development of tactile organs. 
Examples of convergence of form may also be noted, such as 
Marginella and Pseudomarginella among Gastropods, Pholas candida 
and Petricola pholadiformis among Lamellibranchs. 
The duration of life in individual Molluscs is ordinarily rather 
short. Marine Streptoneura may live for several years, and Lattorina 
littorea, when in captivity, has attained an age of nearly twenty 
years. Freshwater Molluscs may live for eight years (Paludina). 
