ALTERNATION OF GENERATIONS IN DOLIOLUM. 479 



resembling that described above for tin- larva on leaving the egg 

 (p. 386). An anterior narrowed portion becomes the ciliated pit, 

 a posterior process changes into an unpaired nerve running from the 

 ganglion, while a third [tart of the rudiment develops into the gang- 

 lion and the sub-ganglionic body. The pericardial sac (p) and the 

 heart seem, as in the larva, to develop according to the type preva- 

 lent among all Tunicates. The genital rudiment (g) degenerates in 

 the lateral buds. It can still be recognised for some time to the left 

 side of the intestinal loop as a mass of cells. In the buds which 

 become sexual individuals this rudiment breaks up into two unequal 

 portions, the anterior smaller part developing into the ovary and the 

 posterior larger portion into the testis. 



The later differences found in the lateral and median buds and the 

 sexual individuals are explained by the valued forms which these 

 finally attain. The median buds and the sexual individuals tend to 

 assume the characteristic barrel-shape, while the lateral buds increase 

 in height and, as mentioned above, adopt the somewhat asymmetrical 

 spoon-shape, owing to the dilation of the atrial aperture and 

 cavity. 



Two more remarkable and insufficiently understood genera, Anchinia and 



I ~k>lehin ia, which, in the structure of their gills, form a transition between 



Pyrosoma and Doliolum, have still to be added. In these genera only parts 



of detached stolons are known, the asexual " nurse " form, developed from 



the egg which produced the stolon, being still undiscovered. We shall have 



to compare these stolons with 



the dorsal outgrowth of the first 



" nurse" generation of Doliolum. 



As a rule the stolon, in Anchinia 



and Dolchinia, consists of a tube 



which, in cross-section, is round 



(the colonial tube, Fig. 247, c), 



and seems to be formed of a q 



Mii^le layer of flat ectoderm-cells. * 



The interior of the tube is filled .,, .-,,- r .- , .. ., , 



r ic 247. — Diagrammatic cross-section through 



with a gelatinous mass, in which the colonial tube (dorsal outgrowth) of 



are embedded mesoderm -eel Is Dolchinia (after Korotneff). c, colonial 



. m . , tube ; a, buds giving rise to tin- sexual 



varying in shape. The external individuals ; -.. zooids. 



surface of the ectodermal tube 



which is covered by a cellulose mantle carries the various buds (a) which, in 

 later stages of their development, seem to grow out on stalks from thickened 

 parts of the ectoderm. The buds are thus here, as in Doliolum, only attached 

 to the exterior of the so-called stolon. They appear irregularly arranged along 

 the stolon, quite young buds being found among others half-developed, and 

 others again fully developed. In a transverse section of a stolon, however, the 



