482 TUNICATES 



by the family Clavelinidae, and Professor Herdman says that 

 they probably represent " a somewhat artificial assemblage 

 formed of those two or three groups of Ascidians which pro- 

 duce colonies, in which the ascidiozooids are so intimately 

 united that they possess a common test or investing mass ". 

 Among the genera are Distoma (with some British species), 

 Leptoclinum, a very common British genus forming crusts under 

 stones at low tide, Botryllus and Botrylloides, as beautiful as 

 they are common. (Plate XXXV., B.) 



The free-swimming, pelagic, compound PYROSOMES or 

 Ascidise Luciae have the form of a hollow cylinder, closed at 

 one end. ' ' The ascidizooids forming the colony are embedded in 

 the common test in such a manner that the branchial apertures 

 open on the outer surface and the atrial apertures on the inner 

 surface next to the central cavity of the colony". (Herdman, 

 Cambridge Natural History, vol. vii., 1904, p. 90.) The Pyro- 

 somes (genus Pyrosoma) (Plate XXXV., C, D, E) are found 

 near the surface, chiefly in tropical seas, and are brilliantly 

 phosphorescent. The compound individual or " gemmarium " 

 is often as long as one's arm, and it may be twice the length 

 of one's body. It is said that fishermen have sometimes used 

 yard long Pyrosomes as beacons at close quarters. At all 

 events, on a dark night they would be better than nothing. 



Order III. Thaliacea or Salpians 



These are free-swimming pelagic forms, either simple or 

 compound, without hint of tail or notochord in adult life, with 

 a permanent clear test strongly or slightly developed, with 

 more or less complete circular bands of muscle round the body, 

 with a quite peculiar respiratory pharynx and peribranchial 

 cavity, with alternation of generations in the life-history, some- 

 times complicated by polymorphism. There are two main 

 types, Cyclomyaria, with complete circular bands of muscle 

 around the body, e.g., Doliolum ; and Hemimyaria, with mus- 

 cular rings usually incomplete, e.g., Salpa. 



In Salpa (Plate XXXV., F; a solitary asexual form buds 

 out a stolon with prolongation of the most of the important 

 organs of the parent. This stolon becomes segmented into a 

 series of young " chain " individuals. Each member of the 

 chain is sexual, and may, whether in the chain or free, produce 



