AND PHTSIOLOGT OF THE TUNICATA. 335 



other, and having their extremities only slightly incurred ; and in 

 another closely related form the organ is dense, large, and some- 

 what quadrangular, with numerous irregular convolutions formed 

 apparently of several loops of the lining membrane. In Stycla 

 tuberosa and S. mamillaris it is large, oval, and disk-like, with the 

 extremities so indistinct as to be scarcely traceable. The reverse 

 of this is the case in Molgula conchilega, in which it is almost 

 crescent-formed, with the extremities very obvious and well turned 

 inwards. And in Ascidia venosa it is still more simplified, being 

 a mere horseshoe-like loop, with the extremities pointed and very 

 slightly incurved. 



It is not easy to assign a function to this peculiar organ — though, 

 from its position at the entrance to the branchial sac, it may be in- 

 ferred that it is of the nature of a special sense, testing the qua- 

 lity of the inhaled water. Taste could be of little use to an animal 

 that has not the power of selecting its food ; but it would seem 

 necessary for the creature to be warned of the approach of 

 aught deleterious in the respiratory currents. The function of 

 this organ is therefore probably more akin to that of smell than 

 of taste. It is certainly of some importance in the economy of the 

 animal ; for it is constantly present, and is usually closely associ- 

 ated with the ganglion. In some species the tubercle rests upon 

 the nervous centre ; and when placed at a little distance from it, 

 a nerve may generally be traced running towards (and in some in- 

 stances having all the appearances of supplying) it. 



The nervous system is in a very rudimentary condition in the 

 Tunicata. There is but one ganglion ; and it is invariably placed 

 between the two respiratory tubes, in a blood-sinus situated be- 

 tween the inner tunic and lining membrane, which sinus, commu- 

 nicating directly with the great ventral channel of the branchial 

 sac, will be well supplied with aerated blood. The ganglion is 

 fusiform, more or less elongated in the antero-posterior direction, 

 and usually a little constricted in the middle, as if composed of two 

 centres. In A. onentula, and in several other species that have 

 been examined, it is partially folded in a much folliculated gland- 

 like substance, and gives off from each extremity three or four 

 nerves, all of which go to the respiratory tubes and to the adja- 

 cent portions of the inner tunic or mantle. A branch from one 

 of the principal nerves has been traced to the branchial tubercle 

 in one or two species. There is no variation of any consequence 

 in the nervous element in any of the forms examined. 



The organization of Salpa is highly instructive ; for in this 



