The
phylum Chordata is a very large group of animals which include the Subphylum
Vertebrata (humans, dogs, cows, whales, ect.), Subphylum Cephalochordata
(lancelets) and Subphylum Urochordata (sea squirts, tunicates and salps).
The latter is the only subphylum that will be discussed here. All members
of this phylum possess a dorsal nerve cord, a notochord and a pharynx
with paired gill slits. The notochord and gill slits only occur in some
species during the early stages of development. There are two classes
of the Subphylum Urochordata: Ascidiacea and Thaliacea. Only Ascidians
are found in Puget Sound. Ascidians can be colonial or solitary and all
are sessile. They have a tunic which encases the body and is attached
to the substrate. The top of the body has two siphons, an incurrent and
an excurrent, where water enters and exits the body. In colonial ascidians,
the excurrent siphon is usually shared with the surrounding animals. Water
is moved through the incurrent siphon into the pharynx by ciliary action
where food particles are trapped in mucous. The water, along with waste
from the digestive tract, is then moved through the slitted walls of the
pharynx into the atrium, or body cavity, and out of the body through the
excurrent siphon. In solitary ascidians, reproduction occurs sexually
in two ways. One, the egg and sperm being released through the excurrent
siphon to be fertilized. Two, the egg and sperm are fertilized internally
using the atrium to brood the developing larvae. In colonial ascidians,
asexual reproduction occurs.
The
phylum Chordata is a very large group of animals which include the Subphylum
Vertebrata (humans, dogs, cows, whales, ect.), Subphylum Cephalochordata
(lancelets) and Subphylum Urochordata (sea squirts, tunicates and salps).
The latter is the only subphylum that will be discussed here. All members
of this phylum possess a dorsal nerve cord, a notochord and a pharynx
with paired gill slits. The notochord and gill slits only occur in some
species during the early stages of development. There are two classes
of the Subphylum Urochordata: Ascidiacea and Thaliacea. Only Ascidians
are found in Puget Sound. Ascidians can be colonial or solitary and all
are sessile. They have a tunic which encases the body and is attached
to the substrate. The top of the body has two siphons, an incurrent and
an excurrent, where water enters and exits the body. In colonial ascidians,
the excurrent siphon is usually shared with the surrounding animals. Water
is moved through the incurrent siphon into the pharynx by ciliary action
where food particles are trapped in mucous. The water, along with waste
from the digestive tract, is then moved through the slitted walls of the
pharynx into the atrium, or body cavity, and out of the body through the
excurrent siphon. In solitary ascidians, reproduction occurs sexually
in two ways. One, the egg and sperm being released through the excurrent
siphon to be fertilized. Two, the egg and sperm are fertilized internally
using the atrium to brood the developing larvae. In colonial ascidians,
asexual reproduction occurs.