THE AUSTRALIAN MUSEUM 



branchial or atrial cavity, which communicates with the 

 exterior by a single dorsal (rarely two ventral) exhalent 

 aperture. The ventral heart is simple and tubular, and 

 periodically reverses the direction of the blood current. 



The leading vertebrate characteristics of the Tunicata are 

 the notochord, the dorsal nervous system, the ventral heart 

 and the respiratory pharynx with gill clefts ; but these all 

 disappear or undergo modification to such an extent during 

 the metamorphosis, that the degenerate adult would not in 

 most cases be recognised as belonging to the Chordata were 

 it not for our knowledge of the life-history. 



Order I. ASCIDIACEA, Blv., 1827. 



This group includes fixed or free-swimming Simple or 

 Compound Ascidians, which, in the adult, are never provided 

 with a tail and have no trace of a notochord. The free- 

 swimming forms are colonies, and the Simple Ascidians are 

 never free-swimming. 



The test is permanent and well-developed ; as a rule it 

 increases with the age of the individual. 



The musculature of the mantle is in the form of an 

 irregular network, there being no regular circular banrls. 



The branchial sac is large and well-developed. Its walls 

 are perforated by numerous slits (the stigmata) opening into 

 a single peribranchial cavity, which communicates with the 

 exterior by the atrial aperture. 



The anus opens into the peribranchial cavity. 



Many of the forms reproduce by gemmation, and in most 

 of them the sexually produced embryo develops into a 

 tailed larva. 



The order Ascidiacea is divided into three sections the 

 Ascidias Simplices, the Ascidiaa Compositag, and the Ascidia3 



