Introduction to A n imal Morphology. 239 



of these aggregated Salpae again develops sexual 

 organs, and from their eggs a second brood of solitary 

 Salpae spring. 



In the compound forms, like Botryllus, a simple 

 egg develops an embryo with one tail and a many- 

 lobed body with a central opening, the cloaca, around 

 which each lobe becomes a persona. 



The egg in Pyrosoma develops an incompletely 

 organised embryo or cyathozooid, which by budding 

 gives rise to four zooids united together by a vascular 

 cord. The tubular heart of the cyathozooid has a 

 peripheral sinus between its two tunics, and is the 

 centre of a social vascular apparatus, whereby the 

 embryo is nourished. 



Three types of aggregation are met with among 

 the Tunicates ist, solitary forms ; 2nd, social zooids 

 connected by a common vascular system ; and 3rdly, 

 compound, united by a fusion of the dermis, but with 

 no internal union. These forms have a single, some- 

 times branched, cloaca for each colony. 



About 300 species are known, some of which (Pyrosomae 

 and Salpae) are brilliantly phosphorescent. The light in 

 Pyrosoma emanate from two lateral cell masses, formerly 

 considered to be ovaries (Panccri). 



They are divided into two orders : 



i. Thaliaceaj (Troschel) free swimming, simple, or united, 



prismatic or cylindrical, with a clear rutis ; the openings at 



cither ll as a l>and in tin- branchial chamber; deve- 



lopmentn. Appendinilaridx tailed, like 



rvx- of other forn iratory organs with 



:*d two tubular spares; ovaries and testes post- 



: hrart d<> 



mouth and mini; 



at opposite pol->; In-art ; .:',!ernatcly ; 



circular muscular i .:. \\ marked; \isrera in a 



