THALIACEA 685 



single external opening of the latter. The gill clefts are in Pyrosoma 

 numerous (up to fifty), tall dorsoventrally, and crossed by internal, 

 though not by external, longitudinal bars. In the Salpida the first- 

 formed cleft persists and becomes in the adult a single gigantic open- 

 ing which occupies the entire side of the pharynx. The Doliolida 

 have a varying number (few in the oozooid, more numerous in the 

 blastozooid) of short openings. 



In Pyrosoma and the Salpida the egg is retained long in the parent : 

 in Pyrosoma it is yolky and meroblastic ; in the Salpida the embryo is 

 nourished through a placenta. Development is direct, the tailed 

 larval stage being omitted; and the buds formed by the oozooid on 

 its stolon (which has a single epicardial tube) hang together for some 

 time as a chain. In Pyrosoma this chain (of four zooids) coils into a 

 circle around the body of the degenerate oozooid [cyathozooidy Fig. 

 480), and its members then bud in such a way as to form a cylindrical 

 colony, closed at one end, composed of blastozooids. This is the form 

 in which the animals pass their free existence, the oozooid never 

 leaving the body of its parent. In the Salpida oozooid and blasto- 

 zooids are alike well developed and free swimming, and the blasto- 

 zooids, of which there is a long chain, though they may coil into a 

 circle (Cyclosalpa), are incapable of budding and eventually break 

 away in groups (Fig. 481). In the DolioHda there is a tailed larva, and 

 the buds formed on the stolon (in which the epicardial tubes remain 

 separate) break free one by one, though they subsequently make 

 attachment to a dorsal process of the mother, by whom they are 

 carried for some time. 



Order PYROSOMATIDA (LUCIAE) 



Thaliacea which have no larval stage; whose oozooid is degenerate 

 and retained within the parent; whose stolon contains a single epi- 

 cardial tube; and whose blastozooids at first form a short chain, but 

 subsequently by budding constitute a cylindrical colony of ascidian- 

 like individuals. 



Pyrosoma (Fig. 479). The only genus. The colonies vary in length 

 from an inch or two to several feet, and are phosphorescent, from 

 which fact the generic name is derived. 



Order SALPIDA (HEMIMYARIA) 



Thaliacea which have no larval stage ; whose oozooid is well formed 

 and free; whose pharynx has no lateral walls, owing to enlargement 

 of the primary pair of gill clefts ; and whose blastozooids are incapable 

 of budding, but adhere as a chain from which they eventually break 

 free in groups. 



