374 
REPRODUCTION — SPERMATOZOON. 
one perform the part of male to the individual already acting as the 
male, while another specimen may perform the female part to the one 
of the original pair acting as female, which acts as male to the new- 
comer ; even then the number of animals taking part may still be 
similarly added to and a chain of animals formed, of which all except 
the terminal individuals will be reciprocally fertilized. 
Some observers have expressed the opinion that some species, as 
Helix aspersa, pair but once, as although 
isolated after a single pairing, they have 
been observed to continue to deposit 
fertile eggs for four successive seasons; 
this phenomenon may, however, arise 
from the retention and continued vitality 
of the spermatozoa or from autofecunda- 
tion, but it is certain that many species 
may 2 )air fre(|uently at short intervals, 
as is occasionally shown to be probable 
by the fragments of several love-darts 
amongst the viscera (f. 671, p. 366), or 
more undeniably by the presence of two or more spermatophores 
within or near the spermatheca. 
Pairing has also been found to occasionally occur not only between 
closely allied sjjecies, as Helix nemomlis and 11. hortensis, but also 
between species usually considered perfectly distinct or even between 
ditfereut genera, as between Pupa and Buliminus. The union of 
Steiiogym decollata and Helix pimiia has been chronicled by Gassies, 
the resultant hybrid progeny, though often scalariform and bizarre, 
usually resembling their mother. 
The Spermatozoon (o-Trep/xa, seed; ^oov, animal) or Zoosperm, the 
essential male element, is a minute highly specialized motile cell, 
resembling an active protozoan or flagellate infusorian and is usually 
a filiform body witli small cephalic enlargement, almost wholly com- 
posed of nuclein or chromatin, and a filiform contractile tail which 
may have a dilated extremity ; it possesses great vitality, enduring 
great extremes of temperature, and may retain its fertilizing powers 
for lengthened periods ; it is paralysed by acids and stimulated by 
alkaline substances. 
In the hermaphrodite species the spermatozoa and ova are developed 
within the ovotestis ; ripening more or less alternately, the spermatozoa 
P'lG. GOl, — Rival Spermatophores 
within the spermatheca of Amalia 
sinueybyi (F^r.). X G. showing the 
result ofa second pairing; the second 
spermatophore finding the appropri- 
ate quarters already occupied, bursts 
through the walls of the sac. 
