Introduction to Animal 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 

 ^%% 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 &^^ 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 

 ombryo 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 (Pyrosomce 

 and Salpae) are brilliantly phosphorescent. The light in 

 Pyrosoma emanate from two lateral cell masses, formerly 

 considered to be ovaries {Panceri). 



They are divided into two orders : — 



I. Thaliaceae {Troschel) — free swimming, simple, or united, 

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

 •either end ; gills as a band in the branchial chamber ; deve- 

 lopment metagenetic. Family i . Appendicularidae — tailed, like 

 the larvae of other forms ; cloacal ; respiratory organs with 

 •one opening and two tubular spaces ; ovaries and testes post- 

 intestinal ; the cutis forms a house or theca ; heart does not 

 reverse its actions. 2. Salpidce — mouth and cloaca opening 

 at opposite poles ; heart reversing its actions alternately ; 

 circular muscular bands usually well marked ; viscera in a 



