ANATOMY AND PHYSIOLOGY. 49 



from that \vhich flows between it and the mantle or inner 

 tunic . 



THE BRANCHIAL TUBERCLE AND NERVOUS SYSTKM. 



That enigmatical organ the branchial tubercle (the anterior 

 tubercle of Savigny) (PI. XX) is situated in the space between 

 the tentacular filaments and the anterior margin of the bran- 

 chial sac, in contact with the upper membrane of the anterior 

 cord or collar, and immediately in front of the upper ex- 

 tremity of the oral or ventral lamina. It is formed of two 

 parallel folds of the lining membrane pressed close together 

 and united at the extremities ; they seem but as one fold, and 

 are bent into a loop Avith the ends turned towards the inhalant 

 orifice, and, inclining inwards, are a little convoluted. Thus 

 the organ assumes a rounded or oval form, rising above the 

 surface to which it is attached as a depressed compact tuber- 

 cular swelling. An opaque white line marks the separation 

 of the two folds, and follows the convolutions to the extremities. 



This is the form that this curious tubercle assumes in Asclilin 

 scabm, A. affinis, a closely allied species, A. mentula, and 

 Pelanaia corruguta. In A. sordida one of the extremities 

 turns inward, the other outward, so that both are bent in the 

 same direction. But more striking modifications occur in 

 some other species. In one allied to A. mentula there are 

 three loops, crowded upon each other, and having their ex- 

 tremities only slightly incurved ; and in another closely related 

 form the organ is dense, large, and somewhat quadrangular, 

 with numerous irregular convolutions formed apparently of 

 several loops of the lining membrane. In Styela tulerosa and 

 $. mamillaris it is large, oval, and disk-like, with the ex- 

 tremities so indistinct as to be scarcely traceable. The reverse 

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

 crescent-formed, with the extremities very obvious and well 

 turned inwards. And in Ascidia venosa it is still more simpli- 

 fied, 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 inferred that it is of the nature of a special sense, 

 testing the quality of the inhaled water. Taste could be of 

 little use to an animal which 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 respira- 

 tory currents. The function of this organ is therefore prob- 

 ably more akin to that of smell than of taste. It is certainly 



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