TUNICATA. 



473 



the point at which, for his needs of description and generalization, he 

 severs an unequivocally natural series of animals from the wide- 

 spread root or base from which it springs. 



The Brjozoa and many Tunicata, after their severally distinct 

 modes of larval locomotive life, become rooted, like plants : some 

 Tunicata^ moreover, form groups of individuals united together by 

 a common organised external integument ; the present order of 

 the class consists, in fact, of compound and simple Ascidians. I 

 shall first demonstrate the organisation of this group by the larger 

 examples of the simple or solitary Ascidians *, which do not essen- 

 tially differ in anatomical structure from the compound species, 

 whose small size renders this a subject for microscopic investigation. 



TUNICATA. 



The exterior tunic of the solitary Ascidian {fig. 178.) is a thick 

 gelatinous or coriaceous elastic substance, adhering by its base or by 

 a long flexible peduncle to some foreign 

 body, and perforated at the opposite end 

 or at the side by two apertures (a and h). 

 The exterior of this tunic is sometimes 

 rough and warty, the inner surface always 

 smooth and lubricous. Microscopically 

 examined, it consists chiefly of a conglo- 

 merate of non-nucleated cells like the pa- 

 renchyma of Cacti ; chemically analysed, 

 100 parts of the tissue, free from ash and 

 water, gives of carbon 45*38, Hydrogen 

 6*47 ; being the same composition as the 

 " cellulose " of plants.f This non-azotised 

 tissue is traversed by large blood-vessels, 

 and towards its inner surface crystals and 

 nuclei are abundant in the clear homo- 

 geneous basis. The lining membrane is 

 composed of a layer of polygonal, nucle- 

 ated, epithelial cells. 



The second tunic is muscular; it ad- 

 heres to the outer tunic at the circumfe- 

 rence of the two orifices, and is connected 

 to it by blood-vessels at a few other points ; elsewhere it is quite free, 

 and the opposed surfaces of the intervening space between the mus- 

 cular and elastic tunics has the aspect of a serous cavity. Its fine 

 fasciculi of fibres are remarkably distinct, and are arranged in two 



Ascidia mammillata. 



* Preps. Nos. 614, 615, 616. 785, 898 b. 998. 1303 c. 

 t IX. p. 34. 4 



