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You are here:   animal list > Botryllus schlosseri




Botryllus schlosseri (Pallus 1766)

 The Star Ascidian

Dylan Moffitt (2011)




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Figure : Dorsal view of a stained section of a functional zooid of B. schlosseri collected on Heron Island, Septmber 2011. Abbreviations: bb, branchial basket; end, endostyle; int, intestine; ps, pharyngeal slits; st, stomach. Scale bar represents 0.5mm

Digestive system

Some of the most distinctive features in the above diagram are part of the digestive system in ascidians. The endostyle is responsible for secreting a mucous net that filters food from the surrounding environment. The pharynx forms a large basket perforated by stigmata which allow water to flow through it and food to be trapped on the mucous net that lies over the pharynx. At the posterior end of the zooid, the pharynx narrows into the oesophagus. The stomach is convoluted to maximise the surface area for food absorption. Waste is excreted from the rectum into the atrial cavity to be removed by the atrial siphon.

Circulatory system

Like most other ascidians, botryllids have open circularatory systems with blood flowing from the heart located in the posterior of the animal, through a network of blood vessels, into various sinuses forming a hemocoel (Mukai et al. 1978). The ends of the blood vessels are called ampullae, and are clearly visible around the periphery of colonies as pigmented outfoldings of the tunic (Grosberg 1987).

All functional zooids in the colony have their own tubular heart which periodically alternates the direction of blood flow, but this reversal is not synchronous for all zooids in the colony (Mukai et al. 1978).

Reproductive System
Each functional zooid in the colony possesses seperate male and female gonads located on either side of the body, in the body wall, immediately posterior to the palleal buds (Mukai and Watanabe 1976). The mature ovum is surrounded by two follicle layers and is attached to the atrial epithelium by an oviduct. Likewise the male testis is held in place by a spermiduct attached to the atrial epithelium. Following ovulation, the egg is kept in a brooding cup in the atrial cavity (Mukai 1977).

Budding Structures
Each zooid in a colony is essentially an aggregation of three distinct blastogenic generation: the functional or adult zooid; the primary bud; and the secondary bud or budlet. For convenience a bud can be classified into 3 stages by morphology (Mukai 1977):

1.    Budding stage: functional zooids in this stage are still sexually immature and produce the primary buds by lateral outfolding of the atrial epithelium

2.    Heart-beat stage: the primary buds have a heartbeat but their buccal and atrial siphons remain closed

3.    Functional stage: the feeding apertures of the bud are opened for filter-feeding