204 PROFESSOR HUXLEY ON THE ANATOMY 



fact, three or four times as long as they are, and is divisible into a broad trilobed base, 

 shaped somewhat like an ace of clubs, and a narrow fringe-like terminal portion. This 

 may be distinguished by the title of the licemal tentacle (PI. XXX. figs. 6, Qa). 



The form, size, and relative position of the oral aperture remain the same i]i ascidio- 

 zooids which have the oral aperture mounted upon a very short cone ; but as the cone 

 enlaro-es, its ha3mal grows faster than its neural side, and finally becomes prolonged into 

 the labial process, which bends over at a right angle to the direction of the axis of the 

 zooid*. Concomitantly with, and apparently as a result of, this development of the labial 

 process, the plane of the oral aperture gradually shifts, until, in the first place, it lies 

 parallel with the axis of the zooid, and then continuing to turn, as it were, on its haemal 

 margin, it eventually takes iip a position perpendicular to the axis of the ascidiozooid 

 again, but exactly the reverse of that which it had at first. The labial process so com- 

 pletely overhangs the oral aperture when this stage is attained, that the free access of the 

 water to the interior of the zooid must, one would think, be somewhat impeded. 



Two very delicate muscular bands, attached to the inner tunic, succeed one another at 

 short intervals behind the aperture of the mouth, within which the buccal cavity rapidly 

 widens, until it attains its maximiim at about the end of the first fourth of the whole 

 length of the zooid. At this point the buccal cavity ends and the pharyngeal or branchial 

 sac commences, the boundary-line between the two being marked by the anterior end of 

 the endostyle and of the epipharyngeal folds, in the middle line of the hsemal side ; the 

 peripharyngeal ridge at the sides, and the ciliated sac on the neural side. On each side, 

 opposite the middle of the peripharyngeal ridge, is the circular patch-like yellowish organ 

 regarded as the ovary by Savigny. 



The iierijiliaryngeal ridge (ciliated band, mihi, Mem. on Sal])a) is a structure which I 

 have found in all the ordinary Ascidians which I have examined. In Fijrosoma it is a 

 sort of ridge or inward process of the inner tunic, less than g^o^h of an inch broad, on 

 which the epithelial lining of the tunic is peculiarly modified, so as to present the 

 appearance of a multitude of transverse rows of elongated corpuscles, each row being set 

 obliquely to the long axis of the band, so as to be inclined from the ha3mal side and 

 behind, forwards and to the neural side. These corpuscles are provided with short 

 and delicate cilia. If the peripharyngeal ridge is traced upwards on the inner tunic, 

 it is found to reach the anterior extremity of the cleft-Hke entrance to the endostyle, and 

 there to pass into a narrow series of similar corpuscles which runs parallel with, and 

 indeed may be said to form the outer part of, the projecting lip or epijiharyngeal fold 

 (' dorsal folds ' of Savigny and others) which bounds the entrance to the endostyle laterally. 

 Arrived at the posterior extremity of the epipharyngeal fold, these prolongations of the 

 peripharyngeal ridge, or, as they may be termed, epipharyngeal ridges, unite with one 

 another and pass down as a single posterior epipharyngeal ridge along the middle line 

 of the posterior wall of the pharynx to the oesophageal aperture, before reaching which 

 the single ridge divides, and its branches soon cease to be further distinguishable. On 



* In the figures given by Lesueur and by Savigny, the axis of the labial process is parallel with that of the body. 

 In most of the ascidiozooids of my specimen, the end of the process is turned towards the hsemal side, but in some it 

 is bent the other wav. 



