1240 



TUNICATA. 



branches enter the trunk, they give off a 

 number of vessels, which enter a trunk along- 

 side of the parent trunk, the combination 

 forming a sort of delta ; in the latter, they 

 leave the trunk singly, and send off* their 

 branches in a radiating direction. At a little 

 distance from the trunks of both artery and 

 vein, the secondary branches become parallel 

 to one another and perpendicular to their 

 primary branches, the more minute divisions 

 following the same mode of ramification. 



Nervous system. This system consists, as 

 in other Tunicates, of a ganglion situated in 

 the substance of the mantle, between the oral 

 and anal orifices. It is globular, and sends 

 off' nervous twigs, firstly, to the respiratory 

 orifice of the mantle ; secondly, to the respira- 

 tory sac, where it begins to exhibit the trans- 

 verse plates; and thirdly, to the anal orifice 

 of the mantle. 



Generative system. The generative organs 

 consist of two elongated tubes, closed at one 

 end, open at the other, and having a great 

 number of close-set parallel cceca arranged at 

 right angles, and opening into them along each 

 side. These tubes are attached to the internal 

 surface of the mantle ; their mouths are free 

 for a short distance, and prominent the rest 

 of their extent, and the attached cceca 

 adherent. The orifices of these organs are 

 situate at the junction of the first with the 

 second quarter of the animal, and one third 

 of the other end of each turns in towards its 

 neighbour, and then proceeds forward parallel 

 to itself. The branchial vein runs midway 

 between the generative tubes above, and the 

 branchial artery in a corresponding course 

 below, so that the threads of the attachment 

 of the plaits on the external surface of the 

 sac are fixed into the tubes in a series on 

 each side. 



From the details of the structure above 

 given, it is evident that the Pelonaia is a 

 true Tunicate. Its anatomy is important, and 

 assists in the elucidation of the nature of the 

 parts and organs in other members of the 

 group. They present the positive anatomical 

 character of a union of the mantle with the 

 test ; so that there can be little question of their 

 right to be regarded as members of a distinct 

 family of Tunicata ; and it is worthy of 

 notice, says Prof. Forbes, that Mr. MacLeay, 

 in his valuable remarks on the arrangement 

 of the Tunicates (Linnaean Transact, vol. xiv.) 

 had hypothetic-ally indicated such a group as 

 this now constituted. Whilst in many of 

 their characters they approach the true As- 

 cidians, especially the unattached species of 

 the genus Cynthia, in others they indicate a 

 relationship with the cirrhograde Echinoder- 

 mata. They differ from their Tunicate allies 

 chieHy by their not being fixed, and by their 

 form, which reminds one more of that of a 

 Siphunculus than of an Aacidia ; indeed, they 

 may be regarded as analogous to certain 

 Siphunculidce ; and in that point of view the 

 details of their form and structure are of 

 much interest to the naturalist. They differ 

 also, add Messrs. Forbes and Goodsir, from 



the Ascidians, more particularly in being bila- 

 teral. The generative organs are symmetrical* 

 and open one on each side of the anus, 

 which is directed towards the ventral surface 

 of the animal, in a line with the mouth and 

 the nervous ganglion. The latter is thus 

 proved to be an abdominal or subcesophageal 

 ganglion, corresponding to or forming one of 

 the chains of ganglia on the abdominal surface 

 of the Articulata. In the same manner, the 

 branchial artery or heart is proved to be the 

 pulsating dorsal vessel, and the branchial vein 

 the abdominal vessel (when that vessel exists) 

 in the Annulosa. It is interesting also to 

 perceive, that, co-existing with this decided 

 approach to the annular type of form, we 

 have the transverse plaits of the respiratory 

 sac corresponding to the rings of an articu- 

 lated animal. The disappearance of a separate 

 test is also a departure from the plan of 

 formation in the Ascidiadce and their allied 

 groups, and an approach to other types of 

 form, and more particularly to the cirrhograde 

 Echinoderms, with certain of which Pelonaia 

 has at least an analogical relation, in the 

 water-filled body, and in the external form. 



Pelonaia, in fine, is one of those connecting 

 genera so valuable as filling up gaps in the 

 system, and supplying links in the chain of 

 structures which runs through the series of 

 organised bodies. 



LOCOMOTION OF TUNICATA. Generally 

 speaking, the muscular tissue of the Ascidiadce, 

 Botryllidce, and Pelonaiadce is not subservient 

 to the purposes of locomotion, and merely 

 effects the sudden removal of water and 

 noxious objects from the branchial sac. The 

 entrance and exit of water through the ex- 

 ternal orifices, constituting the chief sign of 

 vitality in these apathetic creatures, are ge- 

 nerally caused by the ciliary currents. 



But the water may be driven out by an 

 ejaculatory effort of the constrictor muscles 

 of the mantle ; which action would be followed 

 by a more or less rapid expansion or dilata- 

 tion of the mantle and test, effected by the 

 elasticity of the latter. In such species as 

 occasionally occur unattached, or even float- 

 ing about freely, such an ejaculatory action 

 would effect a transitory retrograde move- 

 ment ; but we have no evidence that any of 

 the Ascidiadce or Pelonaiadce make use of this 

 propulsive agent in the manner of the Pyro- 

 somata and the SalpfE. Nearly all the Asci- 

 diadce and Botryllidce are sessile, but some 

 few, mounted on flexible pedicles as Boltcma 

 and Sigillina, may be said to enjoy a limited 

 freedom of movement, necessary perhaps to 

 their well being ; and the same may be said 

 of such species as are attached to the floating 

 branches of flexible algae and corallines. 



According to Mr. F. D. Bennett, except in 

 the action of the sphincter-like membrane, 

 surrounding the open extremity of the com- 

 pound cylinder, there was but very slight 

 evidence of motive power in the specimens of 

 Pyrosoma examined by him. MM. Peron and 

 Lesueur, however, describe a slight retrograde 

 motion observable in this animal. The 



