HOMOLOGIES. 



47 



Now nearly all the changes which we have thus hypothetically supposed to take place 

 in Clavelina, in order to convert its branchial sac into the tentacular crown of Phimatella, do 

 actually occur in other genera of Tunicata, some in one, and some in another. The pre- 

 dominant importance of the transverse over the longitudinal bars of the branchial apparatus 

 in the Tunicata is sufficiently manifest ; in most cases, they are larger and more evident than 

 the longitudinal ; in Pyrosoma, they are not only the better developed, but they alone carry 

 cilia. In this genus, moreover, the haemal extremities of the transverse bars of one side are 

 separated from the corresponding extremities of those of the other by a considerable space, 

 and thus present a marked approach to the open condition which characterises the tentacular 

 crown of the Polyzoa. According to Lesieur, in a species described by him, they even hang 

 free into the branchial sac for some distance from their extremities, an important fact not 

 easily reconcilable with the view that the branchial sac of an Ascidian is nothing more than 

 a perforated pharynx permeated by a vascular network. 



In Doliohiiii, the longitudinal bars actually disappear. 



In Salpa, both longitudinal and transverse bars have disappeared, and the gill consists 

 merely of a tubular rod passing obliquely across the thoracic chamber, and furnished with 

 transverse bands of cilia. It is the homologue of the " branchial sinus " in Clavelina; and 

 the mouth, situated at its posterior and haemal end, is related to it exactly as the mouth to 

 the " branchial sinus " in Clavelina. The following two diagrams will render apparent the 

 relations of the parts in Salpa and Doliolum. 



The relation of the parts in Doliolum is particularly interesting, and of great importance 



Fig. 10. 



Fig. 11. 



Fig. 10. Plan of Salpa. 



a+i 



Fig. 1 1 . Plan of Doliolum. 



a + 6. External and middle tunic united, c c. Internal tunic, d d d. General sinus system, e. Respiratory 



orifice. /. Cloacal orifice, g. Respiratory bars. i. Gill {Salpa) and branchial sinus (Dolhlum). 



m. Languette. }i. Mouth. o. (Esophagus, p. Stomach. q. Intestine. r. Anus. s. Cloaca. 

 u. Ganglion, v. Heart. 



in the determination of the present question. In this remarkable little Tunicate, the gill con- 

 sists essentially of a tubular band, extending transversely across the thoracic chamber, 



