682 



THE INVERTEBRATA 



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 com- 

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

 strong but usually incomplete ventrally and convergent dorsally in 



-f-m. 



Fig. 478. Fig. 479. 



Fig. 478. The asexual form (oozooid) of Salpa democratica-miicronata. From 

 Sedgwick, after Claus. at. atrial opening ; Br. " gill " (hyperpharyngeal band) ; 

 est. endostyle; M. mouth; Ma. test; Nu. "nucleus"; Stn. stolon; Wg. ciliated 

 pit. 



Fig. 479. Pyrosojna. A, A colony. B, The same cut open longitudinally. 



the Salpida (Fig. 478), strong, complete and regular in the Doliolida 

 (Fig. 483). 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. 

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



