MONSTROSITIES — HETEROSTROPHY. 
115 
does not invariably correctly indicate the organization, as it is always 
in proximity to and in close association with the respiratory organs, 
changing its position in correlation with the modifications which the 
breathing organs are so liable to undergo. In Ancylus, which is a 
hyperstrophic species, the respiratory cavity has been lost, and the 
function of respiration assumed by the left margin of the mantle and 
by a secondary pseudo-branchial appendage, which has been developed 
on the same side, the heart having moved in correlation with this 
development from its normally dextral position towards the left side, 
so as to retain its proximity to the functional aerating organs, 'fhe 
embryo of Planorhis corneiis has a spirally sinistral shell, with the 
heart on the right side of the body, thus indicating a sinistral organiza- 
tion of the animal, hut as growth proceeds the heart gradually moves 
dorsally and nearer the left side, probably in correlation with the 
development of an auxiliary hranchia upon the left side, and the shell 
gradually becoming a discoidal and practically dextral species, and 
therefore heterostrophic in character. 
/ 
Figs. 258, 259. — Portion of spire of 
Tiirhonilla y'ufa Phil., showing how the 
change from hyperstrophically sinistral 
to orthostrophicaily dextral coiling takes 
place (after Fischer). 
IlETEROSTROPnic (eVcpos, other [than usual way] ; a-rpocfn'i^ a turn) 
shells are those which do not continue their growth in the same direc- 
tion as they begun, Init at a certain 
period of their existence gradually, 
hut (piickly, change that direction, so 
that a sinistrally organized species may 
commence life as an orthostrophicaily 
sinistral shell, and end as a hyper- 
strophically dextral one ; or, on the 
contrary, maybe at first hyperstrophic, 
that is, a sinistrally organized animal 
may have a dextrally coiled shell, which afterwards becomes sinistrally 
convoluted or orthostrophic and in accord with the organization of 
the mollusk, and vice versa ; yet the 
enrolment of all these forms always 
belongs to the same spiral, which 
may, as pointed out, he at first nega- 
tive and afterwards become positive, 
or the reverse, the differently coiled 
embiyonic portion being often left as though affixed to the side of the 
apex of the spire. In addition to the Planorbes, we have an example 
of these heterogyi-ate forms, as such shells are also called, in A ncylus 
Fig. 2()0. — Apex of shell of Ancylus 
yjuviatilis (Miill.), showing the onho- 
sirophically sinistral coiling of early life, 
which later becomes hyperstrophically 
dextral in character (after Bourguignat). 
