121 



TUNICATA. 



The test is so transparent and colourless, 

 that the interior structure of the animal may 

 be easily studied without having recourse 

 to dissection. There is observable, however, 

 certain yellowish lines traversing the transpa- 

 rent test, which have a granular appearance, 

 and correspond, according to M. Milne-Ed- 

 wards, to the lines of junction of certain parts 

 of the interior. Two of these bands descend 

 vertically, very near one another, throughout 

 the medial line of the ventral surface of the 

 thorax, and are separated by a linear, colour- 

 less, semi-opaque space. A third line rises on 

 the right and left of the test, at the superior 

 part of the thorax, and is directed horizon- 

 tally backwards, describing a circle around 

 the base of the oral orifice. A fourth line, 

 also annular, encircles the inferior extremity 

 of the thorax. A fifth surrounds the anal ori- 

 fice, and is prolonged upwards and forwards, 

 almost to the posterior margin of the oral 

 orifice. There is generally a sixth line, much 

 paler than the others, and more deeply situ- 

 ated, at some distance from the dorsal surface 

 of the thorax, descending vertically from the 

 superior to the inferior ring. 



Towards the middle of the abdominal por- 

 tion of the body the stomach appears, as a 

 small, orange-yellow, oval mass, bearing four 

 yellow vertical lines, like those of the tho- 

 rax. Lastly, close to the stomach, is another 

 orange-coloured spot, formed by a coloured 

 portion of the intestine, and, still lower, the 

 whitish gland-like mass, comprising the gene- 

 rative organs (fig. 768. p). 



The mantle is suspended within the first 

 tunic ; it is membranous and extremely deli- 

 cate. Superiorly it is attached around the 

 two orifices, and inferiorly it terminates in a 

 cul-de-sac. It often presents inferiorly some tu- 

 bular prolongations, sometimes simple, some- 

 times ramified ; which descend towards the 

 base of the animal, and sometimes project 

 into the interior of the root-like processes of 

 the external tunic. Its surface is traversed 

 with divers muscular fibres, some of which 

 are - circular, and constitute the sphincters 

 around the mouth and anus ; whilst others, to 

 the number of nine or ten pairs, arise from a 

 kind of tendinous collar surrounding the 

 mouth, and descend vertically to the inferior 

 extremity of the abdomen. These last muscles 

 serve to shorten or bend the body, and it 

 would appear that it is by the elasticity of the 

 external tunic that they are lengthened again, 

 after having been so contracted, for there do 

 not appear to be any traces of transverse 

 muscular fibres proper to act as antagonists 

 to the vertical fibres. 



In all the thoracic portion of the body a 

 third tunic is present. This, like the pre- 

 ceding, is membranous, and is suspended in 

 a kind of sac formed by the latter ; it adheres 

 to it by the rim of the anal orifice and along 

 the collar around the base of the mouth. 

 Inferiorly this membranous pouch is united 

 in its extent to the two orifices of the diges- 

 tive canal, where for the most part it is con- 

 tinuous with the wall of that tube. Its cavity 



constitutes the thoracic chamber. It encloses 

 the branchial organs, and presents on the 

 dorsal side a free space, which forms a kind 

 of cloaca, ending at the anal orifice of the ex- 

 ternal tunic. Along the medio-ventral line 

 a vertical groove is observable, and at the point 

 where it adheres to the second tunic, between 

 the mouth and the anus, we can perceive a 

 minute tubercle, which is the nervous gan- 

 glion. This thoracic chamber exactly re- 

 sembles the great cavity of the Salpians, and 

 would scarcely differ at all, if the cloaca were 

 shorter, and the anal orifice more distant from 

 the mouth, and directed backwards. 



The branchiae (fig. 768. e) exist as a large 

 membranous band, arising from the dorsal sur- 

 face of the thoracic cavity, below the gangliform 

 tubercle, and by its opposite extremity fixed to 

 the space situated between the cesophageal ori- 

 fice and the termination of the intestine, thus 

 separating the cloacal from the great pharyn- 

 geal or respiratory cavity ; only, in place of 

 presenting on each side simple striae furnished 

 with vibratile cilia, as in the Salpians, this 

 kind of vertical body bears right and left a 

 series of filiform appendages, directed hori- 

 zontally towards the ventral side of the respi- 

 ratory cavity, where they are fixed on each side 

 of the middle sulcns, and, during their passage 

 across, are united together by a number of 

 other slender vertical filaments. From this 

 disposition of the parts there results a kind of 

 trellis-work, which fills up all the pharyngeal 

 portion of the branchial chamber, permitting 

 no communication between the latter and the 

 cloaca, except through the meshes of its net- 

 work, which are bordered all around with 

 vibratile cilia. This complicated branchial 

 apparatus adheres also to the thoracic tunic 

 by its two extremities. The dorsal column, 

 which thus forms the base of the branchial 

 organ, and which represents the simple bran- 

 chia of the Salpians, is considerably prominent 

 in the interior of the respiratory cavity, and 

 exhibits along its ventral margin a series of 

 ten membranous languets, which are usually 

 straight, and are apparently susceptible of a 

 kind of erection. The interior of this mem- 

 branous band is occupied by a large vascular 

 cavity, which M. Milne Edwards terms the 

 branchial or dorsal sinus. The transverse little 

 fillets which spring therefrom are in number 

 about twelve pairs, and the vertical filaments 

 that unite the latter are nearly of the same 

 size : these are about thirty in a rank, and 

 there are thirteen ranks, viz. eleven situated 

 between the two transverse fillets, and two 

 which stretch from the first and second fillets 

 to the wall of the thoracic chamber, and arc 

 fixed along the two yellow lines, placed as 

 rings at the two extremities of that chamber. 

 Some membranous processes appear also to 

 stretch from different points of the surface of 

 the branchial net-work to the walls of the 

 cavity in which the latter is suspended ; but 

 these are few, and do not hinder the eggs, 

 deposited in the cloaca, from insinuating 

 themselves frequently into that portion of the 

 cavity situated on each side of the respiratory 



