The adult larvaceans resemble the
tadpole-like larvae of most
tunicates. Like a common tunicate larva, the adult Appendicularia have a discrete trunk and tail. It was originally believed that larvaceans were neotenic tunicates, giving them their common name. Recent studies hint at an earlier divergence, with
ascidians having developed their sessile adult form later on. As the larvae of ascidian tunicates don't feed at all, the larvae of
doliolids goes through their metamorphosis while still inside the egg, and salps and pyrosomes have both lost the larval stage, it makes the larvaceans the only tunicates that feed and have fully functional internal organs during their tailed "tadpole stage", which in Appendicularia is permanent. The full development of
Oikopleura dioica and the fate of its cell lineages have been well-documented, providing insight into larvacean anatomy. In some genera like
Oikopleura, the tract is U-shaped, with the anus located in a forwards position compared to the stomach and intestine. Others like
Fritillaria present a more segmented appearance, with a straighter digestive tract and well-separated pharyngeal and digestive sections. The species
Appendicularia sicula doesn't have any anus at all, leading to accumulation of undigested material. Appendicularia retains the ancestral
chordate characteristics of having the pharyngeal spiracles and the
anus open directly to the outside, and by the lack of the atrium and the atrial siphon found in related classes. The gonads are located in the posterior section of the trunk, beyond the digestive tract. They are the only section of the body not to be well-distinguished in the juvenile post-tail shift, instead only growing in size in the days leading to spawning.
Tail The tail of larvaceans contain a central
notochord, a dorsal nerve cord, and a series of striated muscle bands enveloped either by epithelial tissue (oikopleurids) or by an acellular basement membrane (fritillarids). Unlike the ascidian larvae, the tail nerve cord in larvaceans contains some
neurons. The tail twists during development, with its dorsal and ventral sides becoming left and right sides respectively. In this way, the dorsal nerve cord actually runs through the tail to the left of the notochord, connecting to the rest of the nervous system at the caudal ganglion at the base of the tail. The muscle bands surrounding the notochord and nerve cord consist of rows of paired muscle cells, or myocytes, running along the length of the tail.
House To assist in their filter-feeding, larvaceans produce a
test or "house" made of
mucopolysaccharides and
cellulose, secreted from specialized cells termed oikoplasts. In most species, the house surrounds the animal like a bubble. Even for species in which the house does not completely surround the body, such as
Fritillaria, the house is always present and attached to at least one surface. The house is secreted from oikoplasts, a specialized family of cells constituting the oikoplastic epithelium. Derived from the
ectoderm, it covers part (in
Fritillaria) or all (in
Oikopleura) of the trunk. In larvae, surface fibrils are secreted by the epithelium prior to the differentiation of the oikoplasts, and have been suggested to play a part in the development of the first house, as well as the formation of the cuticular layer. The houses possesses several sets of filters, with external filters stopping food particles too big for the larvacean to eat, and internal filters redirecting edible particles to the larvacean's mouth. Including the external filters, the houses can reach over one meter in
giant larvaceans, an order of magnitude larger than the larvacean itself. The house varies in shape: incomplete in
Fritillaria, it is shaped like a pair of kidneys in
Bathochordaeus, and toroidal in
Kowalevskia. The arrangement of filters allows food in the surrounding water to be brought in and concentrated prior to feeding, with some species able to concentrate food up to 1000 times compared to the surrounding water. Larvaceans have been found to be able to select food particles based on factors such as nutrient availability and toxin presence, although both laboratory feeding experiments and
in situ observations show no difference in feeding rate between their usual food sources and
microplastics. In some species, houses are discarded and replaced regularly as the animal grows in size and its filters become clogged; in
Oikopleura, a house is kept for no more than four hours before being replaced. In other genera such as
Fritillaria, houses can be regularly deflated and inflated, cleaning off particles clogging the filters. Houses being reused in this manner leads to a smaller contribution in marine snow from these genera. This has been confirmed through genetic studies on
Oikopleura dioica and the
ascidian Ciona, pinpointing their common cellulose synthase genes as originating with a
horizontal gene transfer from a
prokaryote. However, houses and tunics share key differences — while houses are gelatinous and can be deflated or even discarded at will, tunics are rigid structures definitively incorporated into the animal's filter-feeding apparatus. == Ecology ==