AND DEVELOPMENT OF PYROSOMA. 217 



Of tlie outer tunic all that requires to be said is, that it becomes relatively thinner as 

 development goes on. In buds which are situated within a certain distance of the open 

 end of the ascidiarium, and which have attained a length of iVth of an inch (fig. 24), the 

 outer tunic of the neural wall of the atrium is raised into a slight rounded projection (r =), 

 and in older buds (fig. 25) this gradually elongates, and extending towards the open end 

 of the ascidiarium, and finally into the lip of the cloacal aperture, becomes converted into 

 one of the stolons of the test. 



The atrial muscular bands are visible in buds not more than -^oth of an inch in length 

 (fig. 23) ; the pharyngeal muscular bands, only in more advanced zooids. 



The tentacular fringe appears first as an inward thickening of the parietes of the 

 mouth. The hgemal tentacle is markedly the longer, even in such buds as that represented 

 in fig. 24. The ganglion is discernible in buds T^btb of an inch long (fig. 20) as an 

 opake oval mass situated between the peduncle and the oral end of the bud, and very 

 much larger in proportion to the rest of the organism than afterwards. The ciliated sac 

 appears as a short cfecal diverticulum of the pharyngeal cavity, connected with the anterior 

 and liEemal side of the ganglion. 



A most curious structure is visible in buds T^oth of an inch long, and remains obvious 

 until they have attained a length of aVth of an inch or thereabouts. For want of a better 

 name, I will term this the ' diapharyngeal band.' In the section, fig. 21, its upper part is 

 visible, passing obliquely downwards and backwards from between the two middle bands 

 of the endostyle ; while, in figs. 22 & 23, its lower extremity is seen to end in the pha- 

 rynx, immediately over the posterior moiety of the ganglion. The diapharyngeal band is 

 hollow, and effects a communication between the htemal and neural sinuses ; and if, as is 

 possible, the heart of the bud has at this period but little functional activity, the ex- 

 istence of this direct channel may facilitate the circulation of the blood. However this 

 may be, this structure becomes longer and thinner as the development of the bud advances ; 

 and all that remains of it, in buds -jVth of an inch long (fig. 24), is a small tubercle which 

 lies over the posterior part of the ganglion. Eventually even this disappears. 



I have already spoken of the origin of the branchial stigmata. Fig. 21 represents an 

 accidental, but very fortunate, longitudinal section of a bud -7-oth of an inch long ; the 

 razor having passed rather to the right of the middle line above, rather to the left below. 

 As it is seen from the right side, the inner surface of the left wall of the branchial sac is 

 exposed to view. Of the eleven stigmata, those in the middle are the longest and most 

 oval, those at the two ends of the series shortest and most rounded. They look clear in 

 the centre, but on careful examination they are seen to be closed, the sheet of indifferent 

 tissue which forms the innermost wall of the pharynx being continued over them. I am 

 strongly inclined to think that it is this sheet of indifferent tissue which gives rise to the 

 longitudinal branchial bars, for in more advanced buds (fig. 22), in which the median 

 stigmata have undergone much elongation, the same layer is continued over their htemal 

 and neural ends, while it has disappeared in the interval, except along three longitudinal 

 lines, where it evidently forms the foundatiom of as many longitudinal branchial bars. 

 In the more advanced stages, new stigmata are added to the anterior and posterior ends 



VOL. XXIII. 2 G 



