284 Dr. G. J. Allman on the Homology of the Organs 



3. Digestive System. — The form, structure, and peculiar course of the ali- 

 mentary canal in the Tunicata, closely resembles what we find in the Polyzoa. 

 This canal in the Polyzoa consists of three distinct portions : cEsophagus, sto- 

 mach, and intestine ; the oesophagus communicates with the stomach by a well- 

 defined cardiac orifice, and the cardiac extremity of the stomach frequently 

 presents a cylindrical elongation, with the ojsophagus opening into its anterior 

 end ; the stomach is separated from the intestine by a well-marked pylorus. The 

 alimentary canal in the Tunicata is also divided into oesophagus, stomach, and 

 intestine; in some instances these divisions are obscurely marked, but in others 

 they are as well defined as in the Polyzoa. Now if, in accordance with the 

 views attempted to be established in the present memoir, we consider the 

 branchial sac of the Ascidian as the homologue of Jhe tentacular crown of the 

 Polyzoon, we shall have the three regions of the alimentary canal of the one 

 exactly homologous respectively with the three regions in the other. If, on the 

 contrary, the branchial sac of the Ascidian be homologous with the first region 

 — the pharynx or oesophagus — of the alimentary canal of the Polyzoon, then, in 

 order to find a homologue in the Polyzoon for that portion of the canal which 

 intervenes between the branchial sac and the stomach in the Ascidian, and which 

 is without doubt a true oesophagus, differing altogether in structure from the 

 stomach, wherever in the Tunicata the alimentary canal acquires its proper de- 

 velopment, we must take the cardiac prolongation of the stomach in the Polyzoa 

 for an oesophagus, a view not borne out either by its structure or its functions ; 

 for independently of the fact that it is not always present, this prolongation ob- 

 viously belongs to the proper stomach, having, it is true, special muscles some- 

 times developed in it, so as to give it the structiure and ofiice of a gizzard ;* 

 but more frequently being a simple prolongation of the gastric cavity, in no 

 respect differing from the remainder of this cavity either in structure or func- 



developed, the intestine does not open into it, but passes forwards and downwards to perforate 

 the middle and external tunics, and thus open directly outwards. There is consequently here no 

 cloaca. Appendicular ia at a first glance appears to aiford the connecting link between the Tunicata 

 and the Polyzoa; but a little consideration will show that the most important point by which it 

 differs from the normal Tunicata, namely, the absence of a cloaca, is that which also separates it at 

 the greatest distance from the Polyzoa. 



* See " Report on Fresh- water Polyzoa.'" Rep. Brit. Assoc, 1850, p. 310. 



