226 FLOATING ASCIDIANS. PAKT m. 



foremost. The side of each, animal in which the nerve- 

 centre is placed is turned towards the open end of the 

 cylinder, the whole of which is cartilaginous and non- 

 contractile. Each of the Ascidians forming this com- 

 pound creature has its outer and inner tunic united and 

 lined with a vascular blood system, a respiratory cavity 

 of large size completely enclosed by a quadrangular net- 

 work, and digesting organs. The sexes are combined, 

 and they are propagated by buds and single eggs. The 

 Pyrosomidse are gregarious and highly luminous ; vast 

 shoals of them extend for miles in the warm latitudes of 

 the Atlantic and Pacific Oceans, and as soon as the 

 shade of night comes on they illuminate ships with 

 bright electric flashes as they cleave the gelatinous mass ; 

 half a dozen of these animals give sufficient light to ren- 

 der the adjacent objects visible. The intensity depends 

 upon muscular excitement, for Professor Fritz Miiller 

 observed that the greenish blue light of the Pyrosoma 

 Atlantica is given out in a spark by each of the separate 

 individuals ; it first appears at the point touched, and 

 then spreads over the whole compound animal. This 

 species appears in such aggregations in the Mediter- 

 ranean as to clog the nets of the fishermen. 



Salpidce. 



The Salpidse are another family of free-swimming 

 Ascidians. The tunic is perfectly hyaline, the body is 

 somewhat cylindrical, but compressed and open at 

 both ends (fig. 163). The mouth is a slit, the dis- 

 charging orifice is tubular and can be opened and shut. 

 The breathing apparatus is in the form of a ribbon ex- 

 tending obliquely across the cavity of the tunic, the ear 

 with four otolites is in the ventral fold, and the flux of 

 the pale blood is alternate as in other Tunicata. 



The Salpidse are produced by alternate generation, 



