52 



PHYLUM TUNIC ATA (UROCHORDA). 



in the form of an elongated band, but in Cyclosalpa it has the form of a 

 ring of from seven to twelve zooids attached together. The first rudiment 

 of the stolon contains, in addition to the endodermal tube, a mass of meso- 

 derm derived from the mesoderm of the parent in the neighbourhood of 

 the elaeoblast and possibly from the elaeoblast itself. Stolons a little 

 older contain on the ventral side of the pharyngeal diverticulum a mass 

 of cells called the genital cord, a pair of peribranchial tubes, one on each 

 side of the pharyngeal diverticulum, a pair of mesoderm bands placed 

 immediately outside the peribranchial tubes, a nerve tube on the dorsal 

 side, and a pericardial tube ventral to the pharyngeal diverticulum on the 

 right side. 



In addition there are two blood-sinuses, one on the dorsal and the other 

 on the ventral side of the pharyngeal diverticulum ; these communicate 



with each other at the free end 

 of the stolon and proximally 

 with the blood sinuses in the 

 body of the parent. The neural 

 and peribranchial tubes and 

 mesodermal and genital bands 

 are confined to the stolon and 

 are not continuous with the 

 tissues of the parent. Their 

 origin is doubtful : according 

 to Seeliger they are all differ- 

 entiations of the mesoderm of 

 the stolonic rudiment ; accord- 

 ing to Brooks the peribranchial 

 and neural tubes are derived 

 from the ectoderm of the base 

 of the stolon, while Korotneff 

 maintains that the peribranchial 

 tubes are derived from the en- 

 doderm tube. The fate of these 

 structures is as follows : the en- 



dodermal tube becomes the 

 FIG. 44. Transverse section through a young 

 stolon of Salpa (after Brooks, from Korschelt 

 peribranchial tubes ; ec ecto- 



and Heider). 



u lower blood-sinus. 



H tnVp riP 

 nd g ue 



digestive canal, the neural tube 

 ^' ds the ganglion and ciliated 

 pit, the peribranchial tubes the 

 atrial cavity, while the genital 

 cord gives rise to the genital organs, and the mesoblastic bands to the 

 muscles, pericardium, heart, elaeoblast and mesoderm generally of the 

 future zooids. 



In the structure of the stolon and its relation to the parent, and in the for- 

 mation of the organs, the budding of salps is not unlike that of Pyrosoma. 

 The pharyngeal diverticulum clearly corresponds to the epicardial tube. 

 The two processes differ mainly in the much larger number of buds pro- 

 duced by salps before separation from the stolon. 



The formation of the buds takes place in the following way. The stolon 

 becomes marked into segments by transverse constrictions due to unfold- 

 ing of the ectoderm. These are numerous (50 to 100), very close together, 

 and all appear at the same time (Fig. 45). They are the buds which gradu- 

 ally enlarge and develop into the sexual zooids. All the organs of the 



