of the Tunicata and the Polyzoa. 285 



tion. In both the Tunicata and the Polyzoa the intestine is invariably bent on 

 the first portion of the ahmentary tube as it passes forward to the anal outlet ; 

 but there is a curious difference between the two groups in this respect, namely, 

 that while in the Tunicata the first bend of the intestine, as noticed by Mr. 

 Huxley, is always towards the lower side, or that opposite to the ganglion, its 

 whole course in the Polyzoa is as invariably towards the upper, or ganglionic 

 side, a difference, however, in no degree invalidating the homological identity 

 of the parts. The structure of the walls of the alimentary canal in the Tunicata 

 reminds us strongly of that in the Polyzoa. In some Tunicata there is a well- 

 developed liver ; in others, however, this organ is entirely absent, or only repre- 

 sented by a peculiar coloured layer on the interior of the walls of the alimentary 

 canal, exactly as in the Polyzoa. 



4. Circulatory System. — The circulatory system of the Tunicata admits of a 

 very interesting comparison with that of the Polyzoa. The degraded condition 

 of the vascular system in the former, where the heart scarcely advanced be- 

 yond the embryonic condition, is alternately branchial and systemic ; and the 

 undefined or extra-vascular circulation in the whole of the abdominal region 

 conduct us at once to the complete absence of the heart in the Polyzoa., where 

 the circulation — altogether extra-vascular, except so far as the tubular tentacula 

 and lophophore represent a vascular system — is effected by the propulsive action 

 of vibratile cilia. The condition of the cu'culatory system in the Polyzoa has al- 

 ready been quite anticipated in the curious Tunicate genus Pelonaia* where the 

 heart itself has disappeared. The great " sinus system" of the Tunicata, filled 

 with the vaguely circulating blood, has its exact homologue in the perigastric 

 space of the Polyzoa., occupying, like the latter, the interval between the middle 

 and internal tunics. 



5. Muscular System. — The muscles on which devolves the office of the re- 

 traction of the polypide in the Polyzoa are of course absent in the Tunicata., 

 but notwithstanding this, we have some interesting points of correspondence 

 between the muscles of the two groups. In the middle tunic or mantle of the 

 Ascidice there is, as is well-known, a large development of muscular tissue 

 in the form of circular and longitudinal fibres, which give to this tunic its cha- 



* See Forbes and Goodsir in Edinburgh New Phil. Jour., vol. xxxii. p. 29. 



2 p 2 



