206 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



(b) The Opaline or Poison Gland. This gland, situated on 

 the floor of the pallial cavity and discharging into it by 

 numerous apertures, consists of enormous unicellular vesicles, 

 whose contents in stained preparations closely resemble those 

 of the mucous glands of the skin generally. The gland is well 

 developed in specimens not more than a third of an inch in 

 length, occupying a relatively greater area than in full-grown 

 individuals. (See Fig. 21, op. gl.) The fluid excreted by the 

 gland is milky and acrid in nature, and in the Mediterranean 

 species possesses a nauseating odour which was well known to 

 the ancients, and was largely responsible for their abhorrence 

 of the Sea-hare. The foetid smell has not been noticed in the 

 British species. While the purple is very readily exuded by 

 Aplysia under irritation, the milky secretion of the opaline 

 gland is only rarely discharged, and then less copiously than 

 the purple. The secretion is probably both protective and 

 excretory. Mazzarelli states that the opaline gland may also 

 exude purple, but this has not been confirmed by other 

 observers. 



THE DIGESTIVE SYSTEM. 

 (Figs. 8-12.) 



[Cut through the floor of the pallial cavity, being careful 

 not to injure the visceral nerve cords or the paired ganglia 

 which lie immediately beneath. The cut should be made 

 between the common genital aperture and the opaline gland. 

 Continue backwards keeping to the right of the anus. Cut 

 forwards in front of the common genital aperture obliquely 

 towards the middle line between the tentacles. This involves 

 cutting across the external seminal groove. Turn back the 

 flaps of body wall, but do not cut any connections in the 

 abdominal region. The opaline gland now lies on the right 

 of the cut, while on the left, by turning up the sub-pallium, 

 can be seen the reproductive apparatus, the parietal and 

 visceral ganglia, and the anterior aorta. The visceral mass 

 is in the centre. Now cut between the parapodium and the 

 visceral hump, following the course of the horseshoe formed 



