THE EVIDENCE FROM DEVELOPMENT. 227 



with normal gasteropod development in its higher and most typical 

 phases. The eggs of the pond snail are to be found in June de- 

 posited on the under surface of the leaves of water-plants, enclosed 

 in capsules containing a white jelly-like matter. The egg undergoes 

 complete yolk segmentation, and then the " gastrula-stage," with its 

 two layers (Fig. 147, <?<:, en) repeated in all animals from sponge to 

 man appears. The mouth of this sac closes as the young form 

 passes to enter the " veliger stage," in which the body is oval, and 

 possesses a ciliated ridge. This latter stage has also received the 

 name of "trochosphere." Ultimately the "foot" is developed, then 

 the shell appears, and in due time the snail-form is assumed. In the 

 pond snail, as a high form of mollusc, we unquestionably find a 

 "veliger- stage," reminding us of the similar phase in other and 

 lower univalves and in bivalves. It is a noteworthy fact that the 

 land snails and slugs do not show the "velum," notwithstanding 

 their apparent nearness to the pond snail. The suppression of the 

 "veliger- stage" here does not surprise us. On the contrary, we are 

 fully prepared for such lapses and omissions in development by the 

 consideration, already enforced, that altered ways of life must in- 

 evitably produce a changed life-history. Such omissions, in fact, 

 exactly answer the expectation of the evolutionist ; and their absence 

 would indeed prove a veritable stumbling-block to his hypothesis. 

 In the " top- shells " (Trochus\ familiar enough as native species, it 

 may be mentioned that the "veliger- 

 stage" (Fig. 148, A), or that of the 

 " trochosphere," is well represented ; 

 whilst in this stage the embryo is 

 also marvellously like the young of 

 certain worms, and also resembles 

 that of some of the Rotifera or 

 " wheel animalcules." Later on, the 

 " velum " (v) of trochus grows larger 

 (B) and becomes more prominent ; 

 and as the shell develops, the larva FlG - 148. DEVELOPMENT OF TROCHUS. 

 assumes the likeness of the young 



" top-shell." Such a life-history is worth recording, even in a cursory 

 fashion, if only to emphasise the fact that, even in some undoubted 

 univalves, the likeness to lower worms is remarkable. 



Certain other univalves of somewhat different structure from those 

 whose development has just been described, may now be noticed. 

 These latter are the so-called " naked " gasteropods, in which a shell 

 is either rudimentary or wanting altogether. But the curious fact 

 remains that, whether a shell is present or not, these animals inva- 

 riably possess that structure in their embryonic state. This shell, 

 which is thus never destined to be developed, is an illustration of 



Q2 



