In this series we’ve been working our way through the various animal phyla, and by now, you’ve
probably realized the term “jellies” gets thrown around a lot. Only members of phylum Cnidaria,
class Scyphozoa are true jellyfish, which we’ve already covered. This means that the
“comb jellies” of phylum Ctenophora may resemble jellyfish but are completely different organisms.
The name “Ctenophora” means “comb-bearer,” which is how this group of animals gets its common name.
All living comb jellies have comb-like plates of cilia
used for locomotion. These combs are arranged in eight equally-spaced bands called comb rows,
which extend from a few centimeters before the mouth to a few centimeters before the anus.
Ctenophores, unlike the Cnidarians, Placozoans, and Poriferians that we’ve examined so far, have
a distinct mouth and two anal pores. The mouth of a comb jelly leads into a pharynx which empties
into a branching digestive tract that ends in anal pores. Ctenophores normally move their combs so
that the propulsion stroke is away from the mouth, meaning that ctenophores, unlike cnidarians,
usually swim in the direction in which the mouth is eating, while the anus trails behind.
All ctenophores live in saltwater environments, and they can exist from the surface waters
all the way down into the deep ocean. Many species demonstrate vertical migration through the water
column based on day-night cycles, where they come to the surface to feed during the day
and return to deeper waters at night, where many emit flashes of luminescence. During the day,
most are colorless and almost transparent, but their comb rows produce a rainbow-like effect
that is due to the prismatic scattering of light as the combs move. In addition,
a few species can also produce secretions of luminescent ink in order to confuse attackers.
Ctenophores have a range of body types. However, some of the most commonly studied
are the Cydippids, Lobates, and Beroids. Cydippids, like the sea gooseberry,
are spherical with two long tentacles that drag in the water and capture prey in epidermal glue cells
known as colloblasts. These colloblasts contain an adhesive glue that binds to prey.
In addition, some cydippids that feed on jellyfish are capable of incorporating their prey’s stinging
nematocysts into their own tentacles. Lobates, like the sea walnut, have muscular cuplike
extensions of the body that extend beyond the mouth. Their tentacles originate from the corners
of the mouth, and run in convoluted grooves that spread out over the inner surface of the lobes.
Beroids, like the cigar comb jelly, have massive pharynxes lined with large, hardened,
fused cilia known as macrocilia that can be used to tear into prey too large to swallow.
Their large mouths can be sealed shut by forming intercellular connections
when not feeding or when digesting prey. Some other highly modified ctenophore body types
include the compressed and ribbon-shaped “band animals,” like the Venus girdle, the medusa-like
Thalassocalyce, and the flattened, creeping Platyctenida that lack comb rows.
The germ layers of the diploblastic ctenophores are similar to the cnidarians
except that instead of a mesoglea that separates the epidermis and gastrodermis, ctenophores have a
collenchyme that contains the muscle cells and amoeboid cells. In addition, ctenophores are
considered to be biradial since their arrangement of internal canals and tentacles, in many species,
change their symmetry from radial into a combination of radial and bilateral.
Comb jellies have a nerve network which forms a ring around the mouth, and is concentrated
under each comb plate. Though they have no brains, or eyes, they do have abundant sensory
cells that line their epidermis, and a large aboral organ opposite the mouth that functions
as a statocyst, or balance sensory receptor. Most ctenophores, except for 3 known species,
are monoecious, which again means that any individual can create both types of gametes
within its own body. Gonads are stored on the lining of the gastrovascular canals
under the comb plates, and gametes, or sperm and eggs, are released during synchronous spawning.
Eggs are generally fertilized outside of the body, but some species, like the sea gooseberry, use
internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has
occasionally been seen in the sea walnuts, and it is possible that most monoecious comb
jellies are capable of self-fertilization. In any case, the larval forms of most species
are very similar in appearance and behavior to the adults, just smaller. Adult ctenophores will
produce gametes while they have a ready supply of food, but if food grows scarce, they will first
halt production of gametes, then shrink their bodies. If food becomes readily available again,
they will reverse the process by expanding their bodies and then resuming production of gametes.
And with the ctenophores covered, we’ve now discussed all of the non-triploblastic
animal phyla. Let’s move forward and see what we can uncover next.