4 BRITISH MOLLUSCA. 



servatlons must he made upon them in the living state 

 before they can be published. Nevertheless, an out- 

 line of the present extent of our knowledge of British 

 species may be serviceable, as directing attention to a very 

 interesting and but partially explored department of our 

 native zoology, and as furnishing some guide for future 

 researches. 



The Tunicata are IMollusca which have no true shell, but 

 are envelo])ed in a coriaceous tunic or mantle ; whence their 

 name. This is constructed in the form of a sac with two 

 openings, or else is shaped like a tube, of greater or less 

 dimensions, open at both ends. Within the tunic we find 

 the viscera, consisting of well-defined organs of respiration, 

 circulation, and digestion, and a muscular and a nervous 

 system. The branchial organ is usually in the form of a 

 sac, placed at the commencement of the alimentary canal, 

 of which it forms, as it were, the antechamber, and is 

 never arranged in distinct leaflets, as it is in the lamelli- 

 branchiate Conchifera. The circulation of their blood is 

 remarkable, on account of its fluctuations and periodical 

 changes of direction. They have no distinct head, and no 

 organs serving as arms or feet. Sometimes they are free, 

 more usually fixed ; but in all cases free during some por- 

 tion of their existence. Some are simple, some present 

 various degrees of combination ; some are simple in one 

 generation, combined in another. They are all dwellers in 

 the sea. Their various states and structures enable natu- 

 ralists to group them under several well-marked tribes, of 

 most of which we have examples in the British seas. The 

 best classification of them is that proposed by Professor 

 Milne-Edwards, lie divides them into three sub-orders, 

 of which the Salpa, the Ascidia, and the Pyrosoma are the 

 types, and subdivides the Ascidians proper into simple, 

 social, and compound. Of all, excejjt the Pyrosoma, we 

 have British examples. 



These animals attracted the notice of the all-observing 

 Aristotle. Like most philosophic naturalists, the question 

 of the distinction between the animal and vegetable king- 



