THALIACEA 603 



Polyclimim. As Clavelina ; but the zooids are imbedded in a common 

 test with only the branchial and atrial openings at the surface. 



Botryllus (Fig. 452). Colonial; the zooids imbedded as in Poly- 

 clinum; but with pallial budding; and with dorsal lamina, and viscera 

 at the side as in Ascidia. 



Class THALIACEA 



Pelagic Tunicata in which the adult has no tail ; a degenerate nervous 

 system; an atrium which opens posteriorly; a stolon of complex 

 structure ; and gill clefts which are not divided by external longitudinal 

 bars. 



Thus defined, the group includes the Pyrosomatida (Luciae), 

 which are transitional from the Ascidiacea, with which they are 

 usually placed, though by their essential peculiarities they belong 

 here. The three orders of the class differ considerably, though two of 

 them (Pyrosomatida and Salpida) are more nearly related to one 

 another than either is to the third (Doliolida). 



In all, the muscular strands of the mantle are arranged as rings 

 which encircle the barrel- or lemon-shaped body. These are complete, 

 but feeble and present at the ends of the body only, in Pyrosoma, 

 strong but usually incomplete ventrally and convergent dorsally in 

 the Salpida (Fig. 453), strong, complete and regular in the Doliolida 

 (Fig. 458). Their contractions cause (in Pyrosoma, assist) the loco- 

 motion of the animal by driving water from the atrial opening — in 

 the Salpida and Doliolida direct to the exterior, in Pyrosoma (Fig. 

 454) into the lumen of a cylindrical colony and thence through the 

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

 numerous (up to fifty), tall dorso ventrally, 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 

 opening 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 (cyathosooid^ Fig. 

 455), 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 



