56 



VERTEBRATE ZOOLOGY 



and serves to propel the colony through the water. Pyrosoma, a 

 typical member of this group, is found swimming near the surface of 



B 



FIG. 28. The free-swimming colonial ascidian, Pyrosoma. A, lateral view 

 (nat. size); B, view of the open end; C, diagram of longitudinal section; at, atrial 

 pores opening into the central cavity or cloaca, ccl, of the colony; br, branchial or 

 oral apertures opening to the outside; end, endostyle; t, test or tunic; v, velum or 

 diaphragm at terminal opening. (From Herdman.) 



warm seas and is brilliantly phosphorescent. Colonies vary from an 

 inch to upwards to twelve feet in length. 



ORDER II. TH ALT ACE A (SALPIANS) 



These are all free-swimming tunicates that live near the sea surface 

 and are doubtless derived from the free-swimming ascidians. They 

 may be either solitary or colonial. In the typical salpians the body 

 is strikingly like a barrel, open at both ends, but with a partition 

 across near the middle. The resemblance to a barrel is enhanced by 

 the presence of a considerable number of muscle bands that encircle 

 the cylinder like barrel-hoops. They are semi-transparent forms, 

 often beautifully colored, and are capable of making very good prog- 

 ress by forcing water out of the broad cloacal opening by contracting 

 the muscles of that region. 



The structure is well shown in the classic salpian type Doliolwn 

 (Fig. 29) . At the left there is the widely open oral funnel which opens 

 into the broad sac-like pharynx. This has a large number of distinct 

 branchial clefts, an endostyle, and peripharyngeal grooves, but no dis- 



