Reproduction is by sexual means. Both sexes have numerous gonads in the pharyngeal region and fertilisation is external. The females lays 2,000 to 3,000 eggs at a time and males release their sperm into the water. The fertilised egg develops into a planktonic tornaria larva which grows for several weeks until it undergoes metamorphosis by dividing body into 3 sections and sinking to the sea-floor.
Class PTEROBRANCHIA
Pterobranchia was established by Ray Lankester in 1877. Pterobranchia, that has about 20 species, is a class of Hemichordata that live in secreted tubes on the ocean floor and feed by filtering plankton out of the water with the help of cilia attached to tentacles. They are small animals ranging in size from 1 to 12 millimetres in length. Pterobranchs live in much deeper water than the enteropneusts. There are about 30 known living species in the group. These are small, and range from one mm to 12 mm. Examples: Cephalodiscus, Rhabdopleura.
The proboscis is modified into a shield which secretes the collagenous case, which is also used as an organ of locomotion. The collar is modified to produce between 1 and 9 pairs of tentacles or lophophore arms. These arms possess a double row of smaller ciliated tentacles. The tentacles secrete mucous for capturing food particles which travel to the mouth by the beating of cilia. The mucous and the accompanying food particles are then digested.
The trunk is short and sac-like rather than being long and thin, and the digestive tract is U-shaped. The animal’s anus is then on the back approximately opposite the animal’s mouth. The truck ends are contractile and prehensile like a stalk. This stalk is used for support in some species but is joined to a common stolon in colonial species.
Asexual reproduction takes place by budding and often gives rise to colonies starting from a single individual. However sexual reproduction is the normal method of reproduction and it is similar to that in the enteropneusta with external fertilisation. However each animal has only a single gonad and the larva is believed not to be a tornaria.
The pterobranchs also differ from the enteropneusts in the possession of only one pharyngeal slit as in Cephalodiscus and no gill slit in Rhabdopleura. Because these animals are generally very small, there is no problem of respiration which can occur simply across the body surface.
Class PLANCTOSPHAEROIDEA
The class Planctosphaeroidea has only one species, Planctosphaera pelagica, which is known only from its larvae. Tornaria larvae of this species are several times larger than those of other species, 8–25 mm long and almost-spherical and transparent, otherwise quite similar to other enteropneust larvae that have a gelatinous body covered with cilia. However, the epidermis of P. pelagica has two deep invaginations or pouch like structures as well as numerous glands that secrete mucous. Planctosphaera larvae, which occur on the ocean surface, trap microscopic organisms by water currents created by the movement of ciliary bands.
Class GRAPTOLITHINA
What is now considered a distinct class of hemichordates, the Graptolithina or graptolites, are common fossils in Ordovician and Silurian rocks. Most fossil graptolites look nothing more than tiny saw blades. However, well-preserved graptolites can be seen to be tubular in cross-section, with the “teeth of the saw” formed by short open branches of the main tube. Careful study of the microscopic structures of the tubes of graptolites showed that they were very similar to the tubes of pterobranchs. Unlike their sedentary pterobranch relatives, most graptolites are thought to have been planktonic, floating or sinking in water. The spiral shape of body was probably an adaptation to slow sinking. Other graptolites may have possessed gas-filled sacs for floating.