48 



Family VII. Polystyelid^ : — Ascidiozooids not 

 grouped in systems. Branchial and atrial apertures 4- 

 lobed. Branchial sac may he folded; internal longitudinal 

 bars present. The chief genera are — Thylacium, with 

 ascidiozooids projecting above general surface of colony ; 

 Goodsiria, with ascidiozooids completely imbedded in 

 investing mass ; and Clwrizocormus, with ascidiozooids 

 united in little groups, which are connected by stolons. 

 The last genus contains one species, Cli. reticulatus, a 

 transition form between the other Polystyelidas and the 

 Cynthiidte among Simple Ascidians. Tlujlacium is the 

 only British form. 



Sub-order III. AsciDi^ Luci^. 



Free-swimming pelagic colonies having the form of a 

 hc^Uow cylinder closed at one end. The ascidiozooids 

 forming the colony are injbedded 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. The ascidiozooids 

 are produced by gemmation from a rudimentary larva (the 

 cyathozooid) developed sexually. 



This sub-order includes a single family, the Pyrosomid^, 

 containing one well-marked genus, Pyrosoma, with several 

 species. They are found swimming near the surface of 

 the sea, chiefly in tropical latitudes, and are brilliantly 

 phosphorescent. A fully developed colony may be from 

 an inch or two to upwards of four feet in length. Pyro- 

 soma does not occur in British seas. 



