III. 4 



ORGANIZATION OF CIONA 



63 



The vanadocytes contain much sulphuric acid and the metal is 

 associated with a chain of pyrrol rings. This haemovanadin is able to 

 reduce cytochrome but it remains uncertain what part the pigment 

 plays in respiration. The blood turns blue in air but cannot take up 

 more oxygen than can sea water. 



The blood is isotonic with sea water, and ascidians appear to have 

 little or no power of regulating their osmotic pressure; none of them 

 is found in fresh water. They are not even able to colonize brackish 

 waters or those of low salinity. For example, they are rare in the 

 Baltic Sea, from which only six 

 species have been reported. Only 

 one species, Molgula tubifera, has 

 been reported from the Zuider 

 Zee (salinity 8-4 per mille). 



A possible reason for this in- 

 ability to regulate the internal 

 composition is perhaps the need 

 to expose a large surface to the 

 water. There are no tubular ex- 

 cretory organs such as could be 

 used to maintain an osmotic 

 gradient. Ninety-five per cent of 

 the nitrogen is excreted as am- 

 monia. Cells known as nephro- 

 cytes found in the blood and 

 elsewhere contain concretions 

 within the cytoplasm and these 

 may in some cases be stored in an excretory sac until the animal dies. 



There has been much debate as to whether the tunicates possess 

 a coelomic cavity. The heart develops from a plate of cells arising 

 early from the mesoderm and lying between ectoderm and endoderm. 

 This becomes grooved and folded to make the heart itself and the 

 pericardium. The irregular system of haemocoelomic spaces around 

 the pharynx and elsewhere is usually said to consist of 'mesenchyme' 

 and to be derived from the blastocoel and therefore not coelomic, but 

 its walls are mesodermal. The situation is complicated by the presence 

 of a pair of outpushings from the pharynx, the epicardia, or perivis- 

 ceral sacs, which end blindly on either side of the heart (Fig. 34). 

 Berrill and others have suggested that these epicardia may be com- 

 pared with coelomic cavities. Their function in the open condition 

 in which they are found in Ciona is perhaps to allow sea water to 



Fig. 34. Section through base of Ciona, 



showing heart, fit., in pericardium, p., 



and the epicardia, e.p., opening into the 



pharynx, b.s. 



at. atrium; g. gonad; int. intestine. 



