TUNICATA, 333 



outlet (ex.) At the bottom of the hranchial sac is the animal's mouth (m) 

 or commencement of the digestive canal, which ends (at a), near the second 

 external orifice. This digestive system is accompanied by other organs, 

 forming the body of the animal, but it appears only like a thickening of one 

 side of the muscular tunic. 



If the animal presenting this organization be compared with the mussel 

 (represented in fig. 30* p. 53,) or the m^ja (fig. 170, p. 244), it will be seen 

 that each has a test Jined by a mantle and furnished with an inhalent and 

 an exhalent orifice ; in each the respiratory cavity is separated from the 

 channel of the out-going current by a sieve-like breathing organ, and in each 

 the currents are produced and food brought to the mouth by microscopic cilia 

 fringing the pores of the gill. The inhalent orifice of each is guarded by 

 tentacles developed from the mantle* and the exhalent opening is often 

 furnished with a valve to prevent a reversal of the current when the animal 

 expands after one of its occasional spasmodic contractions. 



These points of analogy are so obvious and striking, as to have induced 

 many naturalists to believe in a very close relationship between the Ascidians 

 and bivalve shell-fish. We must, however, hesitate before we assume that the 

 organs which perform identical functions, are themselves identical, ("homo- 

 logous.") ]Mr. Hancock has pointed out (in the excellent memoir just 

 -referred to,) that the branchial sac of the Ascidian is not the anatomical 

 equivalent of the gills of mya, but a portion of the alimentary canal ;t 

 and that the peculiarities of their circulation and mode of reproduction are 

 more in harmony with what obtains amongst the higher zoophytes {Iryozoa), 

 A similar view is expressed by M. Milne Edwards in his memoir on the 

 Composite Ascidians.;}: 



These statements are referred to more particularly, since of late years an 



V. v'. referring to the space between the mantle and the branchial sac, indicate the 

 dorsal and ventral sinuses of Milne-Edwards ; ?«. mouth, at the bottom of the branchial 

 sac ; s. stomach, plaited lengthways; i, intestine, lying between the brachial sac and 

 muscular tunic, on the further side ; a, termination of the intestine ; r, reproductive 

 organ, endinji in the cloaca. 



* These tentacular filaments are not anatomically connected with the branchial sac 

 as supposed by Farre and Owen. See Hancock on the Anatomy of the Freshwater 

 Brijozoa. An. Nat. Hist. vol. V. p. 196. 



t Dr. Farre compared the Ascidian gill to the pharynx of the bryozoa ; but M. 

 Van Beneden and Mr. Hancock consider it homologous with the circle of oral ten- 

 tacles in the retracted or undeveloped bryozoon. 



X The Ascidians have less intimate analogies with the Mollusca, properly so called 

 than is usually believed. They resemble, it is true, these animals in the arrangement 

 of their digestive apparatus, and in some peculiarities of the respiratory system ; but 

 they depart from the Molluscan type in mode of circulation, in the metamorphosis 

 which the fry undergo, and above all, in the singular power which most of them 

 possess, of multiplying by gemmation. In these iatter characters, so very important 

 in a physiological point of view, they closely approach the polypes. (Milne-Edwards, 

 Mem. Inst., France, 1842.) 



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