DOLIOLUM — LARVAL DEVELOPMENT. 



:;s:, 



by means of its long caudal region, and is therefore usually called 

 a Inrm although it is still enveloped in the much-distended egg-shell 

 (m) in which traces of the follicle-cells can be found. We do not 

 know for certain when this egg-shell is cast off. During these pelagic 

 ontogenetic stages in which the Doliolv/m resembles the Ascidian 

 larva, the body is elongate (Fig. 17!) .1) and the middle of it is 

 occupied bj a vesicular ectodermal swelling (eb) caused by the 

 accumulation of a clear fluid. This vesicle divides the body into a 

 posterior and an anterior region. The anterior develops into the 

 young Doliolwm (the first asexual or "nurse" form, the blastozooid) 

 while thi' ectodermal vesicle and the caudal region must he regarded 



r eh. 



Kit.. Ism. 5Toung "nurse" form of DolMum EhrenJb&rgii, with remains of the larval 

 tail (after CJljakin). '■//. chorda : d, so-called dorsal stolon ; e, endostyle ; /;, heart 

 and pericardium ; m, egg-shell ; n. nervous system ; /•. rosette-shaped organ (rudi- 

 iii. -lit of tin: ventral stolon). 



as provisional larval orpins and degenerate later (Figs. 179 />, and 

 1<S(J). The structure of the caudal region corresponds to that of 

 the same region in the Ascidian larva. It consists of a chorda and, 

 laterally, <>f muscle-plates derived from the mesoderm-bands. At the 

 anterior end of the caudal region, a pari of the mass of mesoderm- 

 eells (Fig. L 79, ms") is not transformed into spindle-shaped muscle 

 fibres. Two cell-masses (//) are subsequently given off from this into 

 the ectodermal vesicle, where they break up and change into blood- 

 corpuscles. 



The anterior region of the body contains the very large rudiment 

 of the central nervous system (Fig. 1 7'.» J, n) and the anterior 



