SEA MUSSEL MYTILUS EDULIS. I 79 



visceral ganglion. The cells are devoid of cilia, which differentiates the osphradium 

 from all the other sense organs 



The structure of the abdominal sense organ is similar to that of the pallial sense 

 organ, but it is larger and more complex. It forms a sense hill from three to four times 

 as high as the contiguous epithelium, from which it arises with a steep slope. The 

 transition from the body epithelium to the sense epithelium is sudden, without any 

 cells of an intermediate character intervening. A characteristic feature of the organ 

 is that it is several cells thick. The distal ends of the cells show distinct striations and 

 are covered with a thick cuticula. The innervation of the abdominal sense organ comes 

 from a side branch of the principal posterior pallial nerve. The nerve enters at the 

 base of the sense organ and sends out numerous branches which spread throughout its 

 structure, some of which connect with or surround the nuclei, while others, passing the 

 length of the cells, penetrate the cuticula and project out some distance as tactile hairs. 

 (See fig. 171, p. 180.) 



- The otocyst, when examined under the microscope, is found to lie just under the body 

 epithelium between the cerebropedal and cerebrovisceral connectives. The main 

 body of the otocyst, which is oval in form, is made up of several layers of cells, which 

 inclose a cavity containing numerous small irregularly-shaped particles varying from 

 1 to 4 microns in diameter and which, on account of their insolubility in acid, are probably 

 silicious in character. The anterior and posterior walls are thicker than the others, and 

 the nuclei of their cells are longer. The cavity is lined with a layer of epithelium, each 

 cell of which bears a long tuft of cilia. (See fig. 173.) 



The otocyst canal is made up of a single layer of more or less cubical cells containing 

 small spherical nuclei. These cells bear very fine cilia that, in the beginning of the canal 

 at least, slope inward, preventing the passage outward of any of the contents of the 

 otocyst. The anterior end of the canal opens to the exterior in the bottom of a funnel- 

 shaped pit formed by an invagination of the body epithelium. The cells which form 

 this pit are arranged in concentric layers. The whole otocyst is inclosed in a homo- 

 genous, structureless substance which is thickest about the oval-shaped portion. 



The otocyst nerve, carrying an abundance of fibers, arises from the cerebropedal 

 connective just posterior to its junction with the cerebrovisceral connective and passes 

 backward just under the otocyst canal into the main body of the sense organ. 



Microscopic examination of the eyes of Mytilus edidis reveals the fact that they are 

 formed from an invagination of the ciliated body epithelium. This gives a cuplike 

 depression whose walls are formed from the transformed epithelial cells of the body. 

 These cells have lost their cilia and are filled, especially toward the periphery, with 

 rather coarse granules of brown pigment (fig. 166, p. 178). The transition from optic 

 epithelium to body epithelium is abrupt, but the two groups of cells are covered with 

 a continuous layer of cuticula. Mucus or some other crystalline secretion fills the optic 

 cup and is thought by some to constitute a lens. 



A very delicate nerve arises from the cerebral ganglion and passes directly back 

 to the eye, where it breaks up into numerous fibrillse that spread over and enter the optic 

 epithelium. 



The pigmented epithelium, covering the mantle edge and other parts of the body, 

 is composed in most part of columnar ciliated, epithelial cells which are partially filled 

 with fine, brown granules. These granules lie for the most part in the region distal to 

 the nuclei (fig. 165, p. 178). 



