GENERAL NOTES ON MOLLUSCS. 411 



female. There its discoverers described it as a parasitic worm, 

 " ffetfocotylus" The lost arm is afterwards regenerated. In Sepia, 

 however, the modified arm is not discharged, but is simply thrust into 

 the mantle cavity of the female. The spermatophores probably enter 

 the oviduct, and burst there. 



FIG. 221. Bunch of Septa eggs attached to plant. After Jatta. 



The eggs, when laid, are enclosed within separate black capsules 

 containing gelatinous stuff, but the stalks of the capsules are united, so 

 that a bunch of " sea-grapes " results. 



GENERAL NOTES ON MOLLUSCS 



From the description of these three types a general idea 

 of the structure of Mollusca may be obtained, but it should 

 be noted (i) that all the three types are specialised; (2) 

 that two small classes, the Solenogastres and the Scaphopoda, 

 are unrepresented in the descriptions ; (3) that in the three 

 classes to which the types belong there is much diversity 

 of structure, this being especially true of the large and 

 heterogeneous class of Gasteropods. 



In surveying the structure of the whole group, it is con- 

 venient to begin with the most striking of the external 

 characters the absence or presence of a well-developed 

 head region. 



In the Lamellibranchs or Pelecypoda the head is absent, 

 and along with it the tentacles, the radula, and the 



