448 NOTES ON PROSOIiRANCniATA, 



it, secondaiy shell, the following suggestions are advanced in 

 explanation of the above types of transition. 



In the first type it seems probable that, parallel with the 

 gradual cessation of functional activity on the part of the 

 primitive shell-gland, there was a gradual assumption of secretive 

 activity on the part of the epithelial cells of the mantle and 

 visceral hump. In Mela the acceleration of development which 

 the formation of neanic structure within the egg-capsule presents, 

 points to a correspondingl}' transitory- nepionic stage, con.se- 

 tjuently there is only a slight defining line. It is evident from 

 the weight and size of the neanic shell described above that, as 

 in Neritina and Onchidium, the veliger stage of J/, diadema is 

 passed within the e^^g. In Triphora^ on the other hand, there is 

 an extreme protraction of the Aeliger stage: it has been obtained 

 in mid-ocean with several adult whorls already formed. In this 

 genus the true embryonic shell or protoconch is calcareous. 

 Doubtless the original cap of the smooth nucleus was horny; we 

 are nevertheless not dealing with a pseudoprotoconch, for whilst 

 still a veliger or phylembryo the organism had acquired the ability 

 to secrete a calcareous shell. The explanation given at the 

 beginning of this paragraph still applies, but in this instance the 

 transference of functional activity from the primary to the 

 secondary .shell-secreting area took place some time prior to the 

 nepionic stage. The protracted retention of the velum here allows 

 a much greater development of the other organs, so that the mantle 

 edge assumes its adult form and secretes shell-structure which, 

 although genetically embryonic, is morphologically adult. The 

 other organs being well advanced in development, the nepionic 

 stage isconcerned only with the degeneration of the velum, in these 

 instances also it will be ver}' condensed. From these remarks it 

 ma}'' be reasonably expected that this type of transition will 

 prove to be correlated with an extremely condensed nepionic 

 stage. 



The metamorphosis of organs just mentioned appears at first 

 sight to be nepionic, but as long as the organism retains the 

 velum in full development it is to be regarded as a phylembryo; 



