34 



THE MOLLUSC A 



Stylifer (Fig. 20) ; or, like Eulima 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 Mpllusca. 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, 

 Lamellarta, etc.), in which the 



Fio. 20. 



Stylifer celebensis, x 12. pr, pro- 

 boscis ; ps, pseudopallium ; sp, spire 

 of the shell not covered by the pseudo- 

 pallium. (After Kiikenthal.) 



Fio. 21. 



Entosiphon dnmatis, x 2. o, orifice 

 of the proboscis ; ov, ova ; pr, pro- 

 boscis ; ps, pseudopallium ; s, fixative 

 siphon. (After Koehler and Vaney.) 



individual assumes the colour and aspect of its habitat, as for 

 instance Hermaea dendritica on green algae, and Hennaea 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 

 vnd Petricola pholadiformis among Lamellibranchs. 



The duration of life in individual Molluscs is ordinarily rather 

 short. Marine Streptoneura may live for several years, and Littorina 

 littorea, when in captivity, has attained an age of nearly twenty 

 years. Freshwater Molluscs may live for eight years (Paludina). 



