ON THE ZOOLOGICAL STATION AT NAPLE.S. 103 



transparent membrane, we might say merely the lining membrane to the 

 bony cavity in whicii it is contained. In shape it is triangular, the 

 base being towards the median line of the body ; the dorsum, on account 

 of its close relation to the dorsal plate, is necessarily flat ; the ventral 

 (uspect or floor is somewhat spoon-shaped, being enclosed in a prolonga- 

 tion backwards of the exoccipital boiic. This prolongation also forms 

 the lateral margins of the cavity, and is united to the dorsal plate 

 above. 



This secondary portion of the bladder is therefore entirely surrounded 

 by bone, excepting the foramen, by which it posteriorly communicates 

 with the primary portion. 



Making now a vertical longitudinal section through the primary por- 

 tion of one side, so as to obtain a view of the interior, we see that centrally 

 there is a thin membrane perforated in one place by a foramen, dividing 

 it transversely into two. Immediately behind this membrane there is a 

 tunnel seen passing in a transverse manner from one primary division to 

 the other ; this passes below the vertebral column, and is the only portion 

 situated below. In this way, therefore, the bladder, instead of being 

 composed only of two, is seen to be divided into six compartments. 



A. Moreau ' describes an interesting experiment which he made on 

 Trigla liirundo. Observing two nerves passing to the swimming-bladder, 

 having their origin below the pueamogastric, near to the first dorsal pair, 

 he stimulated them with electricity, and produced the characteristic 

 grunting sounds of the Triglidoe. 



On studying the bladder, he found in the dividing septum (which he 

 calls the diaphragm) radiating and circular muscular fibres tormina a 

 kind of sphincter round the central circular opening. 



On removing the posterior end of the bladder, and stimulatiri"' a^^ain, 

 although naturally no sound was produced, this sphincter was seen to 

 contract. He therefore concludes that the sound is produced by vibra- 

 tions caused by the contraction of this diaphragm. 



While killing two of my specimens of Badylopterus sounds exactly 

 similar to those of the Gurnard wei-e distinctly heard, and simultaneously 

 with eacli sound a distinct contraction of the bladder could be felt from the 

 exterior. These contractions were quite independent of any movements 

 of the mouth or operculum. After examining the diaphragms of Trigla 

 hirundo, T. lyra, and Dadyhpierus, and finding their structures all exactly 

 similar, I am inclined to support Moreau's conclusion. 



Studying now more carefully the bones forming the cavity for the 

 secondary portion, we find that from the basi-occipital the neural arch 

 slopes forwards, while the exoccipitnls have branches projecting back- 

 wards and upwards, spreailing out superiorly so as to form a broad basis 

 of attachment to the dorsal plate. As has been already noticed, these 

 processes form the floor and external margin of the cavity. The basis 

 occipital with its forward projection forms the boundary. The inner side 

 is formed by the paroccipitals, and the roof by a joint arrangement of 

 supraoccipital and parietal. 



The situation of this secondary portion, snrrounded as it is by skull 

 bones, made me at first apprehend a connection with the ear, as is the 

 case in some Silurida>, Characinidoe, Cyprinidce, and Gymnotidaj,'^ and 



' Siir la Voijr des Pohsonx. 



' Gunthur: Jntroductiun to the Stuchj of Fhhcs, 'Organ of Hearing and Air- 

 Bladder.' 



