DOLIOLUM

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CLASSIFICATION OF DOLIOLUM

PHYLUM :- CHORDATA (Notochord, dorsal tubular nerve cord present and gill slits present)

GROUP :- ACRANIATA (No head, cranium or brain)

SUB-PHYLUM :- UROCHORDATA (Marine. Body covered by a thick test. Notochord present only in larval tail)

CLASS :- THALIACEA (Test with circular muscle bands. Free-living, pelagic).

ORDER :- DOLIOLIDA (Complete ring of muscle bands or cyclomyaria present).

FAMILY :- DOLIOLIDAE

Genus : Doliolum

Doliolum is a marine, pelagic, free-living and solitary tunicate swimming by forcing water out of atrium after each powerful muscle contraction.

Doliolum is commonly called as chain tunicate. It measures I to 1.5 cm.
It shows polymorphism and exists in two phases:
1. Gonozooid or solitary phase, and
2. Oozooid or gregaria phase.
Fully grown oozooid has a barrel-shaped transparent body, a large mouth, branchial and atrial apertures at opposite ends and 8 complete muscle bands. Body covered by test and below test is mantle :
1. Edges of above lobes contain sensory lobes, 10 on branchial and 12 on atrial side,
2. Mouth leads into pharynx with a few stigmata. It has ventral endostyle, and peripharyngeal bands. Viscera consists of oesophagus, stomach, intestine, digestive gland, heart and neural complex,
3. Oozooids reproduce asexually by stolon budding.
Life-cycle is peculiar consisting of 1. Sexual form 2. Tailed larva developed sexually 3. First asexual form 4. Second asexual forms (Phorozooid) developed on cadophore and buds developed on stolon.
The animal moves through water by jet propulsion. The contraction of the muscle bands drives water backwards out of atrial sac and so propels the animal.
Gonozooid can be differentiated from oozooid in having 12 and 10 sensory lobes in branchial and atrial openings, 8 muscle bands, cadaphore and otocyst absent and with testis and ovary.

Doliolum is a favourite tunicate. The fertilized egg develops into a free-swimming tailed larva somewhat like that of an ascidian. The larva metamorphoses into a barrel-shaped adult ‘nurse’ stage that by asexual reproduction gives rise to several individuals. Some even produce sperm and ova.