64 Descriptions of Preparations. 



parieto-splanehnic gang-lioii, the nerve of the right side, as well 

 as that of the left, has been brought into view, and a slip of blue 

 paper has been passed under the two nerves just before they enter 

 the ganglion. This nerve-centre is well seen giving off sensory- 

 branches to the mantle, and motor to the adductor muscle, and 

 visceral branches to the gills. Its finer visceral branches given 

 to the organ of Bojanus, and to the anus, are not seen in this pre- 

 paration. Four or five delicate nerves may be seen to pass off from 

 the pedal ganglion into the muscular strata of the foot; to the 

 most posteriorly placed but one of these, the auditory vesicle may 

 be found appended, at a point corresponding pretty nearly with 

 the junction of the anterior two-thirds with the posterior third of 

 the foot, close to the line beyond which the viscera do not extend 

 downwards within their muscular envelope. The branches which 

 the labial ganglion gives to the anterior portion of the mantle, are 

 not seen in this preparation. 



By the division of the transverse commissural floor which united 

 the inner lamellae of the two inner gills across and below the 

 structures already described as being placed posteriorly to the 

 posterior edge of the foot, and by the turning outwards of the flaps 

 thus formed, a view is obtained of the spaces bounded by the two 

 lamellae of each gill, and also of the line of junction of the outer 

 lamella of the inner gill and the inner lamella of the outer gill, 

 on each side to the organ of Bojanus, which sends blood into the 

 lamellae of the gills along their internal surfaces. 



Each gill is seen to be a hollow pouch or sac with its cavity 

 divided into innumerable partitions by strips of tissue which run 

 across from one lamella to another. But for some distance down- 

 wards from the attached or upper border of each gill, these trans- 

 verse bands fail to be developed, and passages are thus left along 

 this border of the gills along which the ova pass in their circuitous 

 course from the reproductive gland to the marsuj^ial pouch which 

 the external gill is formed into for them. The portion of this canal 

 which the inner gill forms is divisible into three segments — an 

 anterior, a middle, and a posterior. The anterior segment is formed 

 permanently by the attachment of the inner gilFs inner lamella to 

 the visceral mass; and it is just within the posterior portion of 

 this segment that in the Anodon the orifice of the generative 

 gland opens. The middle segment is formed temporarily into a 



