REPORT ON THE MARSENIAD^E. 7 



rolled up (Onchidiopsis). It is sometimes broadened (3Iarsenia), sometimes narrowed at 

 the apex (Onchidiopsis). The seminal duct runs along the whole length of the penis, 

 and its end can usually be stretched out in a finger-like protrusion. 



The central nervous system exhibits a structure in general agreement with that of the 

 other az} 7 gobranch streptoneural Gasteropods, 1 and essentially differs from that of some of 

 them (Cassidaria), only in the shortening of the proximal ends of the pleural (visceral) 

 commissure. Each half of the superior ganglionic mass exhibits three ganglia lying in 

 close association ; on the left — a cerebral, a pleural, and (usually on the upper side of 

 them) a supra-intestinal (siphonal ganglion of Jhering) lying on the latter ; on the right, 

 side by side — a cerebral and a pleural, with a subintestinal (abdominal of Jhering) situated 

 behind and beneath them. The cerebral ganglia are connected by a rather short com- 

 missure, and give origin to the usual nerves and to the cerebro-pleural and cerebro-pedal 

 connectives. Similarly, from the pleural ganglia there originate the usual nerves and the 

 pleuro-pedal connective ; the right ganglion is united superiorly and to the left with the 

 supra-intestinal, posteriorly and inferiorly with the sub-intestinal, and through the latter 

 with the left pleural ganglion, which is again connected with the supra-intestinal. It is 

 probable that the sub-intestinal ganglion is further connected with the supra-intestinal by 

 a special commissure included in the pleural (visceral). The supra-intestinal, situated on 

 the left, sends nerves to the gill, to the olfactory organ, and to the left half of the mantle. 

 The subintestinal, situated on the right, supplies principally the digestive system and the 

 right half of the mantle. The relations of the buccal ganglia are as usual. — The inferior or 

 pedal ganglionic mass, lying in close connection with the foot, is linked to the superior by 

 the cerebral and pleural connectives ; the connectives of the right side are, however, usually 

 (Chelyonotus, Marsenia, Marseniella, Marseniopsis) much shortened. From the anterior 

 portion of the pedal ganglia a small round secondary ganglion is always separated off, and 

 from this the anterior part of the foot is innervated. 



The eyes are, as we have mentioned, situated on small knobs (ophthalmophores) at 

 the base of the tentacles. The otocysts are inclosed in fiat, milk-white sheaths, formed 

 of connective substance, and are situated on the side of the pedal ganglia, or somewhat 

 more externally. The Nervus acusticus ascends through the cerebro-pedal connective to 

 the cerebral ganglion. The otolith is large and spherical. 



The bipinnate olfactory organ lies in a special groove in front of the gill, and is attached 

 to the shell-ridge by a fine fold, through which the Nervus ol/actorius runs from the supra- 

 intestinal ganglion. The organ itself consists of a rhachis enclosing the nerve, and of rather 

 laro-e leaflets springing from the rhachis. The skin is well supplied with glands, especially 

 in the mantle. On the anterior border of the foot, there is a small gland, the foot-gland, 

 which opens by the previously mentioned pore. 



1 Cf. Spengel, Die Geruchsorgane u. das Nervensystem d. Mollusken, Zeitschr. f. wiss. Zool., 1881, vol. 

 xxxv. p. 373. 



