The likely importance of this is to stop vibrations that could give away the predator’s
position to its own predators.
(^) Chaetognath reproduction is hermaphroditic, generally protandrous. The ovaries are
in the trunk, the testes in the tail section. Sperm transfer is by spermatophore, but the
transfer, while not well described, is mutual. Spermatophores constructed inside
seminal vesicles on the lateral sides of the tail section fill with sperm through an
aperture in the tail-section wall. They are placed next to the female gonopores of both
partners at the posterior end of the trunk section. Most experts believe self-
fertilization is also possible. Sperm migrate into the tubular ovaries that extend
forward along the trunk wall from the gonopores just anterior to the trunk-tail
segment. Ova are generally in single lines on each side. The sperm, which have a
worm-like structure (Shinn 1997), must burrow through “guard” cells in the oviduct
to reach and fertilize the ova. Species of the Sagittidae (previously the genus Sagitta,
now divided into 12 genera: Parasagitta, Serratosagitta, ...) release eggs freely, but
at least some species of Eukrohnia carry them in marsupial sacs. Hatchlings resemble
simplified adults.
(^) The phylum has received intense recent interest because it appears to have separated
very early from the common ancestor of the Bilateria (i.e. all metazoans except the
Cnidaria and Ctenophora). A consensus of genetic sequences (e.g. Marlétaz et al.
2006; Helmkampf et al. 2008) implies a relation to the protostomes, the major
bilaterian branch that includes arthropods, mollusks and annelids. That is a change
from the long-standing assignment of arrow worms to the other major branch, the
deuterostomes, which includes chordate-like groups, chordates, and echinoderms.
Kapp (2000) has re-evaluated chaetognath development, showing that it differs in
many respects from that of either major group, and Ball and Miller (2006) provide a
catalogue of respects in which chaetognaths differ from all other metazoans. Their
body plan is shown by recently reported and convincing fossils from the Maotianshan
and Burgess Shale deposits to have remained essentially stable since the early
Cambrian (Chen & Huang 2002; Conway Morris 2009). Molecular genetics (Papillon
et al. 2006) have provided a reasonable characterization of the relationships among
chaetognath family groups. A peculiarity of their genetics is a radically reduced
mitochondrial genome compared to nearly all other phyla (Helfenbein et al. 2004).
Aesthetics
(^) All that dry terminology does no justice to the elegance and beauty of planktonic
animals. They are a joy to the eye, and even preserved specimens can fire the
imagination with questions. Why are there fancy curved spines and patches of hairs
on copepod legs apparently used only as paddles? Of what use is an eye on the edge