Salpians are highly modified and peculiar tunicates, living in open sea with specialized reproduction. They are found on surface and down to depths of 200 meters.

GENERAL CHARACTERSTICS OF SALP

Body of the animal is cask-shaped, measuring 1 to 8 cm in length and covered by a thick test. The branchial and atrial apertures are at opposite ends. Posteriorly the test in produced into two tail processes. Below test in mantle.
Salpa exhibits polymorphism and exists in two phases : (i) oozooid or asexual phase, and (ii) blastozooid or sexual phase or aggregate type. Two phases are very much alike, but oozooids are smaller in size and having lesser number of muscle bands. Sexually produced Salpa is called as solitary type or oozooid, while alternate generation produced by budding is called as aggregate type or blastozooid.
Typical oozooid consists of branchial and atrial apertures at opposite ends. A ciliated funnel is present on dorsal side. Nerve ganglion, dorsal lamina and pharynx which has endostyle, peripharyngeal band gill present.
Oesophagus, stomach, intestine and gonad constitute a compact mass along the postero-ventral side which is called as visceral nucleus. Muscle bands incomplete.
A stolon originates from ventral side and it carries endostylic and mesenchymal extensions. Stolon produces buds or blastozooids.
Blastozooid is a sexual phase, having paired testes and a single ovary.
Blastozooid is protogynous. Fertilization is internal. Ovum gives rise to a single sexless free oozooid.
Important character of the animal is locomotion by propelling mechanism in which active muscle bands contract and inturned water is ejected through atrial pore.

SPECIAL FEATURES

Sexual and asexual generations alternate a single adult asexually produces several hundred individuals connected to one another. Small fragments of such a chain consisting of severalanimals, sometimes break loose from parent. Both connected and separated individuals reproduce sexually. Each individual produces one or several eggs that remain connected to parent by a placenta like structure, analogous to that of higher chordates where the maternal and embryonic circulation are closely related during development. The hatching egg becomes asexual form.