CHAPTER XIII 



ASCIDIANS AND LANCELETS 



THE Ascidians (Tunicata), or Sea Squirts, are a class of degenerate 

 survivors of ancestral Vertebrates. Their true character was not 

 understood until 1866, when the discovery of their complete life- 

 history showed that they stood at the base of the Vertebrate series. 

 These animals are enveloped in a leathery tunic or mantle whence 

 the name Tunicata. This tunic is constructed in the form of a 

 sac with two openings, or else in the shape of a tube of varying 

 dimensions. Within this tunic are the viscera, comprising the 

 organs of respiration, circulation, digestion, and a muscular and 

 nervous system. The Ascidians, in their adult stage, have no 

 distinct head and no appendages serving as arms or legs. Though 

 a large proportion are fixed to a base, some are free ; some are 

 simple ; some show various degrees of combination ; others are 

 simple in one generation, combined in another, displaying a regu- 

 lar alternation of generations. All, however, are free during the 

 larval period of their lives, when they present a more or less strik- 

 ing resemblance to the tadpole of a frog, and exhibit unquestion- 

 able vertebrate characters, such as the spinal cord, notochord, 1 



1 " The axis of the backbone of all Vertebrates is formed by an elastic rod known 

 as the notochord, which lasts throughout life in some of the lowest forms, but in 

 the higher forms appears only in the embryo. The universal occurrence of this struc- 

 ture has been regarded as the most important characteristic of the Vertebrata and 

 their allies, which are accordingly grouped together in the phylum Chordata. The 

 members of this phylum are further distinguished from other animals by several im- 

 portant features. Of these one of the most important appears to be the existence of 

 lateral outgrowths of the pharynx, which unite with the skin of the neck and form a 

 series of perforations leading to the exterior. These structures are the gill-slits, and 

 in the fishes their walls give rise to vascular folds or gills. With the assumption 

 of a terrestrial life the higher Vertebrates lost their gills as functional organs, respira- 

 tion being then performed by entirely different organs, the lungs. But even in these 

 cases the gill-slits appear in the embryo, and remains of one pair can usually be recog- 

 nised in the adult state of even the highest Vertebrates. Another fundamental cha- 

 racteristic of the Chordata is given by the central nervous system, which lies entirely 

 above the alimentary canal, just dorsal to the notochord. Not only does this position 

 of the nerve-centres distinguish the Chordata from the Invertebrates, but a further 

 point of difference is found in the development. While in Invertebrates the ventral 

 nerve-cord is formed as a thickening of the ectoderm or outermost layer of the 

 embryo, in the Chordata the nervous system is usually formed as a longitudinal 



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