142 ARTHUR WILLEY. 



mata on each side of the pharynx. In the next stage there 

 are five pairs of stigmata ; and, finally, the last stage of the 

 infantile or nepionic period of develo23ment is heralded by 

 the appearance of a sixth pair. 



The primary branchial stigmata become greatly drawn out 

 in the transverse direction (i. e. dorso-ventrally), and even- 

 tually become subdivided to form secondary branchial 

 stigmata, Avhich, in their turn, undergo elongation in the 

 longitudinal direction, and again become subdivided to form 

 the tertiary branchial stig'mata of the adult. 



In some cases, therefore, it is highly probable that the 

 entire stigmatic system of the advilt is derived by subdivision 

 from six pairs of primary branchial stigmata. 



Thus the nepionic period of development comprises four 

 stages, w^hich it is necessary to particularise, because each of 

 them requires or may require separate morphological treat- 

 ment ; and, moreover, young Ascidians in one or other of 

 these stages may be collected in the open, and so constitute 

 specimens requiring diagnosis. 



The stages may be named and tabulated as follows, atten- 

 tion being again drawn to the singular absence of a stage 

 presenting three pairs of stigmata : 



{1. Distigmatic stage. 

 2. Tetrastiffmatic stage. 

 3. Pentastigmatic „ 

 4. Hexastigmatic „ 



Young Ascidians may often be collected in the hexastig- 

 matic stage. It is often difficult to know what is meant by 

 stages of development, but it is not difficult in this case, the 

 stages being quite definite. 



The above enumeration of stages refers to species belonging 

 to the family Ascidiid^, and also, as I shall proceed to show, 

 to at least one member of the Molgulidse. The table does 

 not apply to such forms as Perophora, Clavelina, Dis- 

 taplia, etc., where, as shown (and the observation is capable 

 of ready confirmation) by the researches of Giard (1872), 



