HOMOLOGIES. 53 



which we have assumed as homologous with the thoracic chamber of the A.scidian ; and the 

 lophophore, which represents the thoracic sinus of the branchial sac of the Ascidian, becomes 

 separated from it just as the branchial sac separates itself from the walls of the thoracic 

 chamber of the Ascidian. 



The development of Salpa has been especially studied by Leuckart.* From the greater 

 simphcity of the respiratory system in the Salpse, it is easier to compare the Salpse and Polyzoa 

 with regard to the development of this system, than it is to institute a similar comparison 

 between the Ascidise and the Polyzoa. Now, the researches of Leuckart fully bear out the 

 views here taken. As in the Ascidians so also here does the first trace of special organs show 

 itself in the formation of an internal closed cavity, the primary sketch of the branchial cham- 

 ber. From the walls of this cavity the rod-like gill is then separated, leaving behind it a 

 space which is to become the cloaca. The alimentary canal, about the same time, makes 

 its appearance ; and it is not until a comparatively late stage of the development tliat 

 the branchial chamber communicates with the exterior by the branchial and cloacal 

 orifices. 



It is plain that, in its essential points, the resemblance here with what we have seen to 

 occur in the Polyzoa is complete. The closed cavity, constituting the early stage of the 

 branchial chamber in the Salpa, resembles even in minute particulars the early stage of the 

 cavity of the tentacular sheath in the Polyzoa, as will be at once evident from an inspection 

 of Leuckart's figures, while the rod-like gill is developed in the interior of the branchial cham- 

 ber, in all essential points just as the lophophore of the Polyzoa is developed within the 

 cavity which is ultimately to become that of the tentacular sheath in these animals. In the 

 Polyzoa the development is continued by the shooting out from the lophophore of hollow pro- 

 cesses which are to become the tentacula. In the Salpa these processes are never developed, 

 and the gill continues in the condition of a polyzoal lophophore, with the tentacula suppressed. 



It appears to me, then, that the points of connection attempted to be established in the 

 present memoir between the Polyzoa and the Tunicata are those which really exist in nature; 

 but there still remain to be considered some other relations of great interest presented by the 

 Polyzoa. We know that the affinities of organized beings are not confined to those which 

 directly unite them to some single allied form. On the contrary, they stream forth from them 

 in various directions, and bring into a community of plan many different types. 



Besides the close affinity between the Polyzoa and the Tunicata, another series of 

 homologies can be pointed out, bringing the Polyzoa into intimate relation with the Brachio- 

 poda. Mr. Hancockf had maintained the resemblance between the arms of the lophophore 

 in the hippocrepian Pol)'zoa and the oral arms of the Brachiopoda, a resemblance singularly 

 striking, and pointing to an important homological relation. In confirmation of this view, he 

 has further called attention to the resemblance between the parieto-vaginal muscles of the 

 Polyzoa and the pedicle muscles of Terehratula, and has compared their action, in one case in 

 closing the orifice of the cell, in the other in closing the valves of the shell. 



The resemblance between the two sets of muscles is certainly obvious, and may possibly 



* ' Zoologischc Uutersuchungen, Zur Auatomie unci Entwickelung dcr Tunicata,' 1854. 

 t On the Anatomy of the fresh-water Bryozoa, by Albany Haucock, Esq. ' Annals and Mag. of 

 Nat. Hist.,' 1850. 



