AND DEVELOPMENT OF PYROSOMA, 235 



number to nine or ten on each side and the middle ones occupy the whole depth of the 

 branchial sac ; but there are, as yet, no longitudinal branchial bars. The mode of origin 

 of the stigmata appears to be just the same as in the buds. 



The nervous ganglion makes its appearance as a thick mass of indiflPerent tissue between 

 the isthmus and the oral aperture ; and the contour of the gastro-intestinal part of the 

 alimentary canal is discernible on that face of the embryo which is nearest the ovisac. 

 The isthmuses have lengthened to -jio^^i of an inch. 



A foetus a^-th of an inch long, whose ascidiozooids had a length of -6-3-rd of an inch, 

 presented the clear profile view of one of the latter, which is represented in PI. XXXI. 

 fig. IB a. The central tube or canal of the first isthmus is obvious, and it opens freely 

 into the branchial sac of the ascidiozooid between the ganglion and the oral aperture. The 

 central canal in question is bounded by the inner tunic of the ascidiozooid, between which 

 and the outer tunic is an interval which is connected, on the one hand, with the sinuses of 

 the ascidiozooid, and on the other, with the space between the two walls of the cyathozooid. 

 The ganglion is very distinct and occupies nearly the whole interval between the oral 

 aperture and the isthmus. 



The diapharyngeal band, already visible in earlier stages, is continued from above the 

 posterior half of the ganglion to the roof of the branchial cavity ; but its proportions are 

 more slender, as it measures axoth of an inch long, by ygVoth of an inch thick. The oral 

 aperture is not open, but its lobed tentacular fringe may be observed, the haemal tentacle 

 being even now distinguished from the rest of the fringe by its length and form. 



Nine or ten branchial stigmata are discernible ; but there are, as yet, no longitudinal 

 branchial bars. The intestine is completely fashioned ; and the elseoblast is large, con- 

 spicuous, and composed of a reticulated tissue. 



In a foetus -aVst of an inch long, with ascidiozooids -g^th of an inch long by eifth of an 

 inch from their oral to their cloacal extremities and nearly hemispherical in form (the 

 flat side of the hemisphere being applied to the combined ovisac and cyathozooid), the 

 isthmuses measure -^oih of an inch in length ; and it is obvious that, while their central 

 canals connect together the branchial sacs, the interspaces between their double walls 

 place the sinuses of the ascidiozooids in communication. There are ten branchial stig- 

 mata, of which the first and last are very small ; and sis or seven longitudinal branchial 

 bars have made their appearance. The mid-atrium is distinctly developed below and 

 behind the gastro-oesophageal part of the alimentary canal. The place of the atrial 

 aperture is indicated by the union of the atrial and outer tunics, in a round spot at the 

 posterior part of the mid-atrium. In fact, the whole zooid is nearly in the same con- 

 dition as the bud represented in PI. XXX. fig. 22. The renal (?) organ has made its 

 appearance as a patch of opake yellowish cells. 



Ascidiozooids ^^nd of an inch long, by ^th from their oral to their cloacal apertures, 

 which form part of a foetus Jg-th of an inch long, and surround the lower half of the 

 combined cyathozooid and ovisac, have ten or eleven stigmata and seven longitudinal 

 branchial bars. The heemal tentacle is well developed, the rudiment of the ciliated sac is 

 discernible, and between the aperture of the central canal of the isthmus and the oeso- 

 phagus are two rudimentary languets. The diapharyngeal band is very slender. The 



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