66 THE CONSTITUTION OF THE GASTROPOD PROTOCONCH, 



is thrown off very early, even while cleavage is still going on" 

 (loc. cit., p. 124.) 



The retention of the velu^n is very much protracted in some 

 species of Tiiphora, which have been obtained as Veliger 

 larvae with "several whorls of the adult shell formed. " The 

 other extreme is presented by those Pulmonata with tran- 

 sient and poorly-developed Velum. 



Turning now to the very constant absence of the Nepio- 

 conch as a recognisable constituent of the Piotoconcli. It 

 must be clearly understood that this term is applicable not 

 to some particular portion of the protoconch but to any por- 

 tion formed during a particular period in the life-history of 

 the species, namely, the period of transition from embryo 

 to juvenescence, during which the velum becomes aborted. 

 Owing to the variation in the degree of development of other 

 organs at the time of abortion of the velum, the N ectoconch 

 may be formed entirely by the undifferentiated ectoderm of 

 the visceral hump, or in part also by a free mantle-fold, and 

 moreover may be either corneous or calcareous. The Nepio- 

 conch when recognisable, or, in fact, whenever present, is 

 formed by a free mantle-edge, and consequently may be 

 either corneous or calcareous like the N ectoconch. 



In the case of very many marine gastropoda, the length 

 of retention of the velum is probably to some extent condi- 

 tioned by the length of the pelagic existence ; once the larva 

 comes to rest, the velum is probably rapidly aborted. The 

 Veliger of C yiitatiuvi is undoubtedly pelagic, but young 

 specimens having but one half whorl of adult structure added 

 to the Nectocoiich have lost all trace of the velum. Two 

 main types of X ectoroiirli are recognisable, the one corneous, 

 the other calcareous. In the former case, together with the 

 abortion of the velum, there is, during the nepionic stage, a 

 change in the mode of functioning of the mantle ; under these 

 conditions one would expect the secretory activities of the 

 shell-gland to be in abeyance. As a matter of fact, we usually 

 find that there is an abrupt change from the corneous N ecto- 



