XVIII 



RESEARCHES INTO THE STRUCTURE OF 

 THE ASCIDIANS 



British AssociaiioH Report, 1852,//'. 2, pp. 76-77. 



The author stated that he was desirous of laying before the Section 

 an account of some investigations into the structure of the Ascidians 

 which he had been led to make while endeavouring to form a catalogue 

 of those contained in the collection of the British Museum. 



The Ascidians, varied as they are in external appearance, present 

 certain general anatomical uniformities, which are capable of being 

 represented by a diagram. To such a hypothetical structure thus 

 represented, the author gives the name of the Archetypal Ascidian. 

 From this every actual form can be shown to be derived, by verj' 

 simple laws of modification. The author particularly desired it to be 

 understood that he attached no other meaning to the term Archetype 

 than that thus defined. 



It has been a matter of dispute which is the dorsal and which the 

 ventral side of the Ascidians ; there can be no question, however, that 

 the heart is upon one side of the axis of the body, and that the 

 nervous ganglion is upon the other; to avoid all ambiguity, therefore, 

 the author proposes to speak of the " haemal " and of the " neural " 

 sides, in accordance with the nomenclature proposed by him in a 

 memoir ' Upon the Homologies of the Mollusca,' read before the 

 Royal Society. The Ascidian Archetype differs from all others in 

 the following points : 



1, The intestine is always flexed towards the haemal side. In 

 the Polyzoa it is flexed towards the neural side, as pointed out by 

 Professor Allman. 



2. The tentacles are small, while the pharynx is very large, and 

 serves as a respiratory cavity, its parietes becoming perforated. The 

 author combated the view that the " branchial sac " of the Ascidian 

 answers to the tentacles of the Polyzoon, or to the united gills of the 



