32 



CON KLIN. [Vol. XIII. 



noch ziemlich spat, noch nach Anlage des Fusses und der 

 Schalendriise anzutreffen und konnen bis auf die Beriihrungs- 

 stelle ganz normal ausgebildet sein." 



Such double or multiple formations sometimes occur in 

 Crepidula and many other prosobranchs, though so far as I 

 have observed they never reach so advanced a stage as Hey- 

 mons mentions. Among the later stages in Crepidula the ab- 

 normal forms are sometimes nearly like the normal ones, the 

 chief difference being due to irregularities of form, which often 

 take the shape of protrusions or wart-like prominences. A 

 more marked form of degeneration is shown by those embryos 

 which have divided into two or more pieces, each of which may 

 move about independently. It is not an uncommon thing to 

 see a little embryo consisting largely of velum and foot, and 

 entirely disengaged from the yolk cells, swimming actively 

 about in the most amusing fashion. Still greater degrees of 

 degeneration are shown by small fragments, which move about 

 rapidly and are nothing more than little masses of ciliated cells. 



It might be considered that in all such cases these abnormal 

 forms were the result of unfavorable conditions, such as im- 

 perfect aeration, varying density of the sea-water, or rough 

 mechanical treatment, were it not for the fact that in some 

 forms {e.g., Neritina) even under the most perfectly normal 

 conditions a definite number of abnormalities are always found. 

 McMurrich ('86) has given a pretty complete series of forms 

 showing the varying tendency to produce abnormalities which 

 different species possess. In Fulgur and Urosalpinx all the 

 eggs are said to develop ; in Purpurea floridana all do not de- 

 velop, but a considerable number (not definitely stated) break 

 down and are used as food : in Purpurea lapillus there are five 

 hundred to six hundred eggs in a capsule, only twelve to thirty 

 of which develop, while in Neritina fluviatilis there are seventy 

 to ninety eggs in each capsule, only one of which undergoes 

 regular development. In each of these cases the eggs which 

 do not develop, break down and are used as food by the normal 

 embryos. Such cases cannot be accounted for by assuming 

 merely that the environment is unfavorable. Such a cause 

 would give no such definite results as are said to exist, e.g., in 



