parent_taxon pleurogona
A salp ( plural salps ) or ___MASK23___ ( plural salpae or salpas ) is a barrel - shaped , planktonic tunicate . It moves by contracting , thus pumping water through its gelatinous body . Salp jet propulsion is one of the most efficient in the ___MASK72___ kingdom . The salp strains the pumped water through its internal feeding filters , feeding on phytoplankton . The dorsal nerve cord is one of the embryonic features unique to chordates , along with a notochord , a post - anal tail , an endostyle , and pharyngeal slits . The dorsal nerve cord is a hollow cord dorsal to the notochord . It is formed from a part of the ectoderm that rolls , forming the hollow tube , compared to other ___MASK72___ ___MASK35___ , which have solid , ventral tubes . The dorsal nerve cord is later modified in vertebrates into the central nervous system which is composed of the brain and spinal cord . Dorsal nerve cord is mainly found in subphylum ___MASK20___ . A ___MASK50___ is a marine invertebrate ___MASK72___ , a member of the subphylum Tunicata , which is part of the ___MASK5___ , a phylum which includes all animals with dorsal nerve cords and notochords . The subphylum was at one time called Urochordata , and the term urochordates is still sometimes used for these animals . Some tunicates live as solitary individuals , but others replicate by budding and become colonies , each unit being known as a zooid . They are marine filter feeders with a water - filled , sac - like body structure and two tubular openings , known as siphons , through which they draw in and expel water . During their respiration and feeding , they take in water through the incurrent ( or inhalant ) siphon and expel the filtered water through the excurrent ( or exhalant ) siphon . Most adult tunicates are sessile , and are permanently attached to rocks or other hard surfaces on the ocean floor ; others , such as salps , doliolids and pyrosomes , swim in the pelagic zone of the sea as adults . Various species are commonly known as sea squirts , sea pork , sea livers , or sea tulips . Invertebrates are animals that neither possess nor develop a vertebral column ( commonly known as a " backbone " or " spine "), derived from the notochord . This includes all animals apart from the subphylum ___MASK20___ . Familiar examples of invertebrates include ___MASK99___ ; crabs , lobsters and their kin ; snails , clams , octopuses and their kin ; starfish , sea - urchins and their kin ; and worms . A chordate is any ___MASK72___ belonging to the phylum ___MASK5___ , possessing a notochord , a hollow dorsal nerve cord , pharyngeal slits , an endostyle , and a post - anal tail for at least some period of its life cycle . The ___MASK5___ , together with sister clade ___MASK67___ , form the deuterostomes as in the embryo development stage the anus forms before the mouth . A zooid or zoöid is a single ___MASK72___ that is part of a colonial ___MASK72___ . Zooids are multicellular ; their structure is similar to that of other solitary animals . The zooids can either be directly connected by tissue ( e . g . corals , ___MASK95___ , Siphonophorae , Pyrosome or Ectoprocta ) or share a common exoskeleton ( e . g . Bryozoa or Pterobranchia ). The colonial organism as a whole is called a " zoon " , plural " zoa " ( from Greek " zòon "  meaning ___MASK72___ ; plural " zòa ",  ). ___MASK96___ ( commonly known as the ascidians or sea squirts ) is a paraphyletic class in the subphylum Tunicata of sac - like marine invertebrate filter feeders . Ascidians are characterized by a tough outer " tunic " made of the polysaccharide cellulose . In ___MASK72___ anatomy , the notochord is a flexible rod made out of a material similar to cartilage . If a species has a notochord , it is , by definition , a chordate . The notochord lies along the anteroposterior (" head to tail ") ___MASK48___ , is usually closer to the dorsal than the ventral surface of the ___MASK72___ , and is composed of cells derived from the mesoderm . The notochord has been observed to have many functions including developmental functions . The most commonly cited functions are as a site of muscle attachment , vertebral precursor , and as a midline tissue that provides signals to the surrounding tissue during development . Vertebrates comprise all species of animals within the subphylum ___MASK20___ ( chordates with backbones ). Vertebrates represent the overwhelming majority of the phylum Chordata , with currently about 64 , 000 species described . Vertebrates include the jawless fish and the jawed vertebrates , which include the cartilaginous fish ( sharks and rays ) and the bony fish . ___MASK76___ is an order of tunicates in the class ___MASK96___ . The group includes both colonial and solitary animals . They are distinguished from other tunicates by the presence of folded pharyngeal baskets . This provides the etymology of their name : in ancient greek , means the " fold " of a cloth . Stolidobranchian sea squirts are also characterized by the complete absence of an abdomen . The abdominal organs of other tunicates are instead located to one side of the pharyngeal basket in this group . In biology , a phylum ( plural : ___MASK35___ ) is a taxonomic rank below kingdom and above class . Traditionally , in botany the term division was used instead of " phylum ", although from 1993 the International Code of Nomenclature for algae , ___MASK65___ , and plants accepted the designation " phylum ". Depending on definitions , the kingdom Animalia contains approximately 35 ___MASK35___ , Plantae contains about 12 , and ___MASK65___ contains around 7 . Current research in phylogenetics is uncovering the relationships between ___MASK35___ , which are contained in larger clades , like Ecdysozoa and Embryophyta . Pleurogona is an order of tunicates which is no longer in use as it contains the same families as ___MASK76___ . One of the more invasive species of this order is the `` stalked ( or leathery or solitary ) sea squirt '' Styela clava . Another invasive genus include the `` chain sea squirts or chain tunicates '' of the genus Botrylloides and the `` golden star tunicate '' Botryllus schlosseri .