THALIACEA. 103 



the latter (endostylic cone). Sexual reproduction and gemmation 

 take place in the same individual. 



The egg develops within an ovarian sac into an embryo, which has 

 the form of a stunted Ascidian-like individual (cyathozooid), and 

 produces, by budding from a stolon, a group of four individuals 

 (ascidiozooids). The peculiar mode of origin of these individuals 

 has been minutely described by Huxley and Kowalevski (fig. 565 b). 

 The process of budding, by which the colony is increased, is no less 

 complicated : it takes place on a germ-stock (stolo prolifer, fig. 

 565 a, St) placed behind the endostyle. Each commencing bud 

 receives a prolongation of the rudiment of the oviary,* as well as 

 of the endoderm. 



The Pyrosomidce derive their name from the bright light which 

 they emit. According to Panceri, this light proceeds from a paired 

 group of cells ii. the region of the mouth. 



Fam. Pyrosomidae. These animals were discovered by Peron in the Atlantic 

 Ocean, and were at first regarded as solitary individuals. Pyrosoma Per. ; 

 P. atlanticum Per. ; P. elegans and giganteum Les., from the Mediterranean. 



Class II. THALIACEA.t 



Free-swimming transparent Tunicata with cylindrical or cask- 

 shaped body. The mantle apertures are terminal, and at opposite 

 ends of the body. The branchice are band-shaped or lamellar, and the 

 viscera are compressed together into a nucleus. 



The Thaliacea (fig. 566 a } b) are transparent, cylindrical or cask- 

 shaped animals, of gelatino-cartilaginous consistency ; they are either 

 solitary, or the individuals are united in chains (usually in double 

 rows). They move on the surface of the sea by the rhythmically 

 alternating contraction and dilatation of the branchial cavity. The 

 two openings are placed at opposite ends of the body ; the mouth at 

 the anterior end, the atrial at the posterior end, near the dorsal 



* [Generative blastema, or indifferent tissue from which the reproductive 

 organs of the parent were developed (Huxley).] 



f Compare Th. Huxley, " Observations upon the anatomy and physiology of 

 Salpa and Pyrosoma, together with remarks upon Doliolum and Appendicu- 

 laria." Phil. Trans., London, 1851. 



K. Leuckart, " Zoologische Untersuchungen." Heft II., Giessen, 1854. 



C. Gegenbaur. " Ueber den Entwickelungscyklus von Doliolum nebst 

 Bemerkungen liber die Larven dieser Thiere." ZeitscTir. fur iviss. Zool., 

 Tom. VII. 



C. Grobben, "Doliolum und sein Generationswechsel, etc." Arbciten aus 

 dem Zool. Institute in Wien, Tom. IV., 1882. 



