The anatomy of Lottia gigantea Gray. 49 



most braDch of the anterior pallial nerve, close to its point of de- 

 parture from the spindle muscle. The nerves to the right osphradium 

 are numerous and depart from the osphradial ganglion. They are 

 best understood from the figure. 



Just at the inner edge of the mantle where the forward branch 

 of the osphradial nerve (R.Par 1 ) joins the anterior pallial, in the 

 forward angle between the two, there is present a very curious ring nerve 

 (Rg. N), lying in the upper wall of the pallial hood. (The hood con- 

 sists of a dorsal and a ventral wall joined by numerous trabeculae. 

 Through the spaces thus formed, blood circulates and is aerated.) 

 The shape of this remarkable nerve is best shown by the drawing. 

 Small nerves from the right anterior parietal (R. Par'), which is merely 

 a prolongation of the osphradial, supply the ring, which also gives off 

 extremely fine nerves. A second nerve ring (Rg.N') much smaller 

 than the first was found near the median line, in a similar position 

 with reference to the mantle and mantle hood. This median ring is 

 connected with the lateral one by a very fine nerve. Slender nerves 

 are given off from it to the mantle hood, and mesially from the central 

 ring a few faint branches can be seen going toward the circumpallial 

 (which they doubtless either directly or indirectly ultimately join). 

 The lateral ring is larger in smaller specimens than in large ones, 

 and the median ring (Rg. N') could be found only in specimens about 

 two-thirds adult size. Is it probable that we have here a sense organ 

 characteristic of the younger stages of the adult animal? 



By way of emphasizing the fact it may be repeated that the os- 

 phradial nerve of the right side is connected with the pallial nerves 

 in two places and by two nerves — namely the anterior and posterior 

 right parietals (R. Par' and R. Par). 



The left osphradial nerve arises from the supra-intestinal 

 ganglion of the visceral commissure (Sup. I) and follows a course on 

 the left side similar to that of the right osphradial nerve on the right. It 

 is much stouter than the right osphradial, and is at first free, but enters 

 the dorsal head wall just before reaching the osphradium. Like the 

 right, the left osphradial nerve does not go directly under the os- 

 phradium, but behind it, and gives off branches to the organ. These 

 are necessarily small and the figure does not do justice to the finer 

 branches, either in extent or numbers. Where these nervelets are 

 given off there is a ganglion in the osphradial nerve, just as is the 

 case on the right side. After leaving the osphradial ganglion the 

 main nerve enters the thick muscles of the dorso-lateral portion of 



Zool. Jahrb. XX. Abth. f. Morph. 4 



