58 



PHYLUM TUNICATA (UROCHORDA). 



attachment is an epithelial one ; they perforate the tunic and their ecto- 

 derm becpmes adherent to the columnar ectoderm of the dorsal side of 

 the stolon (Fig. 52). By the growth of the stolon in girth they become 



displaced outwards and give 

 rise to the lateral buds (Fig. 

 52, G) of the dorsal stolon. 

 As the stolon is continually 

 growing in length the buds 

 which first affix themselves 

 are carried away from the 

 body and new buds are at- 

 tached between them and 

 the base of the stolon. Thus 

 it happens that the buds 

 increase in age as we travel 

 from the body along the 

 stolon. 



The probuds which arrive 

 first at the dorsal process 

 give rise to lateral buds (6r). 

 Those which arrive later 

 give rise to median buds, 

 which are attached nearer 

 the middle line and are not 

 displaced so far outwards 

 (Fig. 52, P). The fate of 

 the lateral and median buds 

 is as follows. The lateral 

 buds develop into small 

 doliolum - like zooids with 

 the rudiments of gonads, 

 which however soon atro- 

 phy, and with a well-de- 

 veloped alimentary canal, 

 but without a closed atrial 

 cavity (Fig. 52, G). They 

 actively take in nourish- 

 ment, and serve for the nutri- 

 tion of the growing stolon. 

 They are therefore called 

 gastrozooids. They are in 

 osmotic relation, through 

 their epithelial attachment 

 to the dorsal stolon, with 

 the blood of the parent. 

 This relation is of some im- 

 portance, because the a- 

 sexual parent (oozoite) has 

 lost its pharynx and diges- 

 tive canal by atrophy and, 

 retaining only its nervous 

 system, heart, and muscles, 

 has become converted into 



FIG. 51. Dorsal view of the posterior part of an 

 asexual form (nurse) of Doliolum showing the migra- 

 tion of the probuds (after Barrois, from Korschelt 

 and Heider). I lateral buds ; m median buds ; ms 

 5-8 four posterior muscle hoops ; p pericardium ; 

 r rosette-shaped organ ; st ventral stolon ; 'st' dorsal 

 process ; u probuds wandering on the ventral side of 

 the nurse ; u' probuds on the dorsal side, wandering 

 to the dorsal process ; u" probuds on the dorsal 



