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IN VERTEBR AT A. INTERMEDIATE TYPE. THE TUNIC ATA. 



Structure of the Tunicata Explained The Throat or Gullet serving as the Breathing Organ Curious Ebb and Flow of the 



Blood Their Division into Simple, Social, and Compound Ascidians Known to Aristotle 1. SIMPLE ASCIDIANS 



Muscular Nature of Tunic " Sea-Squirts " Where Found 2. SOCIAL ASCIDIANS Mode of Union Genera 3. TRUE 

 COMPOUND ASCIDIANS Their Anatomy 4. THE PraosoMiD^E Their Pelagic Habits Their Phosphorescence 5.. 

 SALPID.E Pelagic Solitary or in Chains. 



THE Tunicata are enveloped in a coriaceous (or leathery) tunic or mantle; whence their name. 

 This is constructed in the form of a sac with two openings, or else in the shape of a tube of greater 

 or less dimensions, open at both ends. Within the tunic are 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 ante-chamber, and is never arranged in distinct leaflets, as it is. 

 in the leaf-gilled bivalve Mollusca. 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 r 

 but in all cases free during the earlier portion 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 naturalists 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. He 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, except the 

 Pyrosoma, there are 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 kingdoms had for him 

 great a-ttractions. The Ascidian was one of the 

 many creatures which he examined, in the hope of 

 gaining definite information respecting such distinc- 

 tion. Its inei't and sponge-like form, rooted to- 

 the ground, seemed to indicate a vegetable nature ; 

 but Aristotie was not content with a mere external survey. He explored its internal structure, 

 and soon perceived its highly animal condition. His description of the Ascidia is wonderfully 

 correct ; it occurs in the fourth book of his ' History of Animals.' There he distinctly recognises the 

 Ascidians to be Mollusca, of which he says ' they are the only kind whose whole body is enclosed in 

 a shell, and that shell of a substance between true shell and leather; it may be cut like dry 

 leather.' What comparison could be more graphic or more true 1 ' They are attached to rocks by 

 their shell. They have two separate openings, which are very small and difficult to notice, one to take 

 in and the other to eject the water. .... If we open them, we find a nervous membrane 

 lining this leathery case, and fixed to it at two points corresponding to the openings, one of which 

 may be looked upon as the mouth, the other the vent.' And then he makes further remarks on their 

 anatomy. His appreciation of the nature of the Ascidians is an interesting proof of the wonderful 

 sagacity and minute observation of the great Father of Natural History." (Forbes and Hanley, 

 "British Mollusca.") 



FAMILY L ASCIDIAD.E (SIMPLE ASCIDIANS). 



The body is sac-shaped, gelatinous or leathery, fixed at one extremity and free at the other ; it has. 

 two more or less prominent orifices, one the "oral," or mouth-opening, the other the "atrial," or excur- 

 rent aperture. The simple Ascidians are not united into groups by a common integument ; but at times 



STRUCTURE OF TUNICATE A, VERTICAL ] B, TRANSVERSE 

 SECTION. 



a, the Atriat or Excurrent Chamber; 6, the Branchial Sac; 

 Stomach : t\ the Test ; t, the Muscular Coat or Mantle ; g, Noi 

 Centre. The Arrows indicate the direction of the Currents. 



