150 AETHUE WILI,EY. 



said is enough to show that the larva of Molgula manhat- 

 t en sis is well worth further study, and the teratological side 

 should not be neglected. I now pass on to the description 

 of the nepionic stages, which is the special purpose of this 

 paper. 



1. Distigmatic Stage (PI. 9, fig. 3).— Owing to the 

 opacity of the larvae the earliest stage at which the two 

 first-formed stigmata could be recognised is that which is 

 represented in fig. 3. In this case the details of the neuro- 

 hypophysial system could not be made out in the object 

 viewed as a whole. It is noteworthy that we have here a 

 median dorsal atrial aperture in place of paired apertures, an 

 observation which accords with that of Julin on Litho- 

 nephria. 



Now if we follow the growth of the two stigmata, which we 

 may distinguish by the letters A and B, Ave find that they 

 increase in size principally in the dorso-ventral direction, A 

 being generally somewhat in advance of B (fig. 4). Having 

 attained a certain transverse diameter, their ventral ends 

 (i. e. the ends directed towards the endostyle) become bent 

 backwards towards the fundus of the pharynx (fig. 5). Next 

 the recurved ventral ends of the stigmata A and B double 

 round in the dorsal direction so that each of the originally 

 simple clefts become biramous, having a larger anterior and a 

 smaller posterior arm (figs. 6 and 7). 



2. Tetrastigmatic Stage (fig. 8). — A young Molgula 

 manhattensis collected in the open is shown in fig. 8, 

 illustrating the commencement of the tetrastigmatic stage. 



Here we note that the posterior arm of stigma A has 

 become separated by constriction from the parent cleft, while 

 in the case of B the abstriction of its posterior arm has not 

 yet taken place. This specimen was fortunately in a state of 

 complete expansion when the sketch was taken, and it pre- 

 sents the facies of a typical Ascidian. I took special pains to 

 note that the stigmata were in precisely the same condition 

 on the right side as that in which they are here represented 

 to be on the left. 



