ASCIDIACEA— ORGANISATION OF THE FREE-SWIMMING LARVA. 371 



muscle-fibrils in which transverse striatum can be distinctly seen. 

 An endocardium is wanting in the Ascidian heart, and its vessels 

 have no endothelial lining. 



E. Review of the Organisation of the Free-swimming 

 Larva iFigs. 168-173 A). 



It should be mentioned that there is considerable variation in the 

 degree of development attained by the organisation at the moment 

 of hatching in the different species and even in the individuals of the 

 same species. 



The form of the larva, the development of which has just been 

 traced in detail, recalls somewhat that of a tadpole. Anteriorly, 

 there is an oral region (cephalic and trunk-region) the end of which 

 carries the three adhesive papillae (Fig. 173, hp) ; this region is 

 followed by a long, flatly compressed swimming tail. This latter, 

 which shows markings like those of fin-rays (Fig. 173 A) attains its 

 characteristic form through the great development of the mantle- 

 substance (Fig. 169, Jl) which further covers the whole surface of 

 the body, even passing over the oral and cloacal apertures. 



The axis of the tail is occupied by the chorda which extends 

 anteriorly into the trunk-region. Above it lies the neural tube, 

 while a cell-strand that runs below it and soon disintegrates repre- 

 sents the remains of that part of the intestine which belongs to the 

 caudal region. Running along this part of the body and extending 

 along its whole length, are the massive muscle-bands. 



In the anterior region, the neural tube swells out to form a sensory 

 vesicle (Fig. 173, sb) and a swollen trunk-section (>■) immediately 

 following the latter. The primitive enteron gives rise to the 

 pharynx or branchial sac, the oesophagus, the stomach and the 

 intestine. The oral aperture (or inhalent orifice, i) is established 

 and is distinguished by its dorsal position. Near it, the ciliated pit 

 (fl), which extends as far as the base of the sensory vesicle, opens 

 into the pharynx. The intestine opens into the atrial cavity which 

 has formed by the union of the two originally separate peribranchial 

 sacs. There is now a single atrial aperture (or exhalent orifice, e). 

 The number of gill-slits which have at this stage developed varies 

 in different species. 



The heart {It), the pericardium and the epicardium (ep) have 

 developed. The heart already pulsates ; the endostyle (hypobranchial 

 groove) has developed. The mesenchyme becomes differentiated into 



