CLASSIFICATION OF THE TUNICATA. 639 



numerous transverse vessels separated by narrow slits (the stig- 

 mafa) and numerous internal longitudinal bars. 



Tentacles present ; simple. 



Dorsal lamina in tbe form of laiiguets. 



Alimentary canal placed posteriorly to the branchial sac. 



Gonads situated in the wall of the peiibranchial cavity, pos- 

 terior to the braachial sac. The embryo becomes a rudimentary 

 larva (the cyathozooid), which gives rise to the first ascidiozooids 

 of the colony. 



Gemmation takes place from a ventral and posteriorly-placed 

 stolon. 



This family contains the single genus Fyrosoma, which may be 

 characterized as follows : — 



Ptrosoma, Feron, 1804. 



Colony free-swimming, and having the form of a cylinder with 

 a large axial cavity closed at one end and open at the other. 



Systems — only one jjresent, the terminal aperture of the colony 

 being the single common cloacal opening, and the axial cavity 

 the common cloaca. 



Ascidiozooids elongated antero-posteriorly, and placed in a 

 single layer with their anterior ends external and their posterior 

 ends internal. Branchial apertures anterior, opening on tlie 

 surface of the colony. Atrial apertures posterior, opening into 

 the ceutrally-placed common cloaca. Body not divided ex- 

 ternally into thorax and abdomen. Apertures not lobed. 



Test gelatinous and transparent, containing numerous stellate 

 branched cells. 



Brancliial sac well developed, not folded, and not extending to 

 the posterior end of the body. Vessels of two kinds — trans- 

 verse vessels which are numerous and closely placed, leaving 

 elongated slits between, which are directed transversely to the 

 antero-posterior axis of the body ; and longitudinal vessels, 

 which are not quite so numerous, and cross the transverse vessels 

 so as to form small quadrangular meshes. 



Tentacles simple, one ventrally placed larger than the rest. 



Dorsal lamina represented by a series of eight or more taper- 

 ing languets. 



Dorsal tubercle with a simple aperture. 



Alimentary canal short and simple, placed posteriorly to the 

 branchial sac. 



