BV H. L. KESTEVEN. 56 



If it be granted that the organism is still a Phylemhryo 

 so long as it is in possession of the larval organs, then the 

 succeeding or Nepionic stage may be defined as "that during 

 which the larval organs are aborted or shed." Such a defini- 

 tion is of perfectly general application ; by it the Nepionic 

 stage in the Ascidians, Amphibians, Lepidoptera, Nemer- 

 teans, etc., is immediately made recognisable. 



As a particular form of this definition applicable to the 

 Gastropoda, I have proposed the following (29) : "That stage 

 during which the velum undergoes degeneration and (more 

 or less completely) disappears." The stage is not generally 

 recognisably represented in the protoconch, and this fact calls 

 for special comment ; but before proceeding to this question, 

 the following extracts from Korschelt and Heider (34, p. 133) 

 are given as illustrating the absence of any necessary relation- 

 ship between- this stage and the escape of the embryo from 

 egg or maternal organism. 



"The ■perfeoily developed V eligtr larva is found almost ex- 

 clusively among the marine Gastropoda, the young of which 

 swim about freely for a long time. Among fresh- water 

 Gastropoda Neritina passes through a stage with a well- 

 developed bilobed velum resembling that of V ermetus, but 

 the Veliger larva does not live a free life, but passes through 

 the stage within the egg-capsule. When the embryo leaves 

 the egg-capsule it shows the adult form." "Onchidium , a 

 Pulmonate living between the tide marks . . . while still 

 within the egg-capsule becomes a Velige?- larva with a coiled 

 siiell and a large bi-lobed velum." "The velum may still per- 

 sist after the foot has attained a considerable size, and when 

 the development of the other organs is also far advanced, but 

 it gradually diminishes in size and finally degenerates, the 

 larva thereby passing over into the adult form which, indeed, 

 had already been nearly approached." 



That Faludina is viviparous, and that most Gastropoda 

 deposit the eggs immediately on fertilisation is so well known 

 as to need no special quotation. In Faiella "the egg-envelope 



