DOLIOLIDA. 



57 



which eventually ruptures and allows the larva to go free. The 

 anterior end of the body assumes the barrel-shaped form of the 

 adult, with its dorsal process, but still preserves its larval tail 

 and notochord (Fig. 48). These eventually atrophy, persisting 

 for a time as a stump, which resembles in structure and position 

 the organ called elaeoblast in the salps and Pyrosoma. 



FIG. 49. Dorsal region of a larva of Doliolum Mullen (after 

 Uljanin, from Korschelt and Heider). cl atrium ; ft dorsal 

 tubercle and duct of subneural gland ; m muscle-hoops ; n 

 ganglion ; nb branchial nerve. 



The principal features in the development are as follows. Cleavage is 

 total and is followed by an invaginate gastrula. In the next stage observed 

 the embryo appears to contain three cell-groups which constitute the rudi- 

 ments of the nervous 

 system, the mesoderm 

 and the notochord. 

 The alimentary canal 

 and endoderm are 

 formed later as a se- 

 condary invagination 

 of ectoderm. The 

 atrium is formed by 

 another invagination 

 of ectoderm. The 

 nervous rudi ment 

 elongates. Its median 

 part remains bulky 

 and gives rise to the 

 ganglion of the adult 



and the subneural gland (ventral lobe) ; its anterior end narrows, acquires 

 a lumen which opens into the pharynx, and forms the duct of the subneural 

 gland and the dorsal tubercle ; the posterior portion also becomes narrow 

 and persists as the posterior unpaired nerve (nervus branchialis). The 

 pericardium is developed as an excavation of a portion of the mesoderm, 

 and the stolon is formed as a small ventral process. 



The stolon in its earliest stage in the embryo consists of a small mass of 

 mesoderm applied against the ectoderm in the neighbourhood of the heart. 

 This is very soon reinforced by two pairs of out- 

 growths, one from the atrium and the other from 

 the pharynx. These, carrying with them the 

 mesoderm, cause a projection of the ventral body 

 wall, which extends into, but does not pierce, the 

 tunic. Later the stolon pierces the tunic and pro- 

 jects freely, and the five strands contained in it be- 



FIG. 50. Probudof Dolio- come increased, by development of the cloacal and 

 lum showing its trans- pharyngeal tubes, to seven. The free end now 

 j^nm^rom" Demand segments into small bodies (Fig. 50), which become 

 H6rouard). c trans- detached and are known as probuds. The probuds 

 body^&r^probuT 11 "' contain a portion of the seven cell masses sur- 

 rounded by ectoderm. They wander by means 



of the pseudopodial activity of certain of their ectoderm cells (Fig. 50, ca) 

 along the right side of the animal to the dorsal side of the base of the dorsal 

 process (Fig. 51). Here they divide into from fourteen to twenty buds, 

 which attach themselves on each side of the middle dorsal line. This 



CCL 



