AND DEVELOPMENT OF PYROSOMA. 203 



and tlie whole ascidiaiium may be regarded as a succession of tiers of ascidiozooids 

 enveloped in a common test. 



The extreme apex of the cone (PL XXX. fig. 5) is formed by only four ascidiozooids 

 ranged round a common point. In the next tier there are at least twelve, and the number 

 increases until, in the widest part of the ascidiarium, there are between thkty and forty 

 in a tier. It should be understood, however, that there is nothing very regular in the 

 arrangement of these tiers, and that the zooids in any given tier are of very various 

 sizes and degrees of development. 



The Ascidiarium presents for study (1) the ascidiozooids, and (2) the common test which 

 envelopes them*. 



The Ascidiozooids. — In investigating the structure of the ascidiozooids, an example 

 from the middle region of the ascidiarium may most conveniently be selected for study. 

 Such an ascidiozooid is represented in longitudinal section in PL XXX. fig. 1, m. trans- 

 verse section in fig. 2, and from above and partly in section in fig. 3. 



It is somewhat irregularly fusiform, a good deal longer than deep, and deeper than 

 broad. Its outer extremity exhibits the oral aperture, which lies upon the hsemal side of 

 one of the above-mentioned conical protuberances, and is overhung by a tongue-like 

 process of the test — the labial process, by whose outgrowth, indeed, its relations and 

 appearance have become so completely altered, that it will be better to become acquainted 

 with the character of the oral aperture in a less modified specimen. On examining one 

 of those oral apertures, in fact, which are hardly, or not at all, raised above the general 

 level of the outer face of the ascidiarium, the plane of the oral aperture is seen to be 

 perpendicular to the axis of the body (taking a line drawn from the oral to the cloacal 

 aperture as that axis). A circular sphincter, composed of a band of unstriped muscular 

 fibres, surrounds the oral entrance, being attached where the lining membrane of the ali- 

 mentary tract (inner tunic : see note, p. 202) and the integument (outer tunic) pass into 

 one another. The inner diameter of the circular sphincter is xioth of an inch ; but the 

 diameter of the oral passage itself is far less, amounting to not more than xwo^li of 

 an inch. This results from the circumstance that the test is thickened at the margins of 

 the mouth, so as to diminish its aperture to this extent ; and it is at the same time 

 puckered, so that when viewed from without, a number of fine grooves appear to radiate 

 from the lips of the aperture. These must not be confounded with certain fine fibres 

 which radiate from the outer margin of the sphincter into the test, and are perhaps 

 muscular (PL XXX. fig. 6). 



The test ceases to be traceable upon the walls of the oral cavity a little within the 

 sphincter ; and where it ends, the inner tunic is produced inwards into a broad fold ^vith 

 lobed edges, which takes the place of that circlet of tentacles which is found in this posi- 

 tion in most other Ascidians. I shall therefore term this the tentacular fringe. It is 

 divided altogether into thirteen lobes, of which twelve, though irregular, are tolerably 

 similar and roughly symmetrical, while the thirteenth is situated in the middle of the 

 hsemal half of the circlet, and is very different in form and size from the rest. It is, in 



* In the present memoir I propose to confine myself as nearly as may be practicable to anatomical and embryolo- 

 gical details, reserving the many interesting histological peculiarities of Pyrosoma for a future occasion. 



2 E 2 



