176 BULLETIN OF THE BUREAU OF FISHERIES. 



position of the osphradia and abdominal sense organs (fig. 169). The osphradia are 

 always found on the inner side of the gill supports, while the abdominal sense organs 

 are on the outer side. List (1902) finds these organs best developed in Modiolaria 

 marmorata and Modiolus barbaratus, well formed in Lithophagus lithophagus , and less 

 developed in Mytilus galloprovincialis. 



Otocysts were first observed in Mytilus by von Ihering (1876), the position, size, and 

 innervation of which he described briefly. List (1902) found the otocyst present in all 

 the Mytilidffi of the Mediterranean region as a paired, symmetrical organ, which he 

 states is always a pear-shaped pustule lying directly under the body epithelium between 

 the two connectives which bind the cerebral ganglion with the pedal ganglion, on the 

 one hand, and with the visceral ganglion, on the other. In Mytilus edulis the organ 

 occupies this same position in the angle formed by the union of the cerebrovisceral and 

 cerebropedal connectives. It lies approximately over the point where the oesophagus 

 joins the stomach. In gross structure the otocyst is an oval body, which in mussels 

 about 5 cm. long has a length of from 150 to 200 and a breadth of from 125 to 135 

 microns. The smaller end points anteriorly and gives off a canal having a diameter of 

 25 to 30 microns, which runs forward just beneath the epithelium for a distance of 700 

 to 1,000 microns, where it opens to the exterior in a funnel-shaped invagination. In 

 very thin specimens a fine white nerve can be traced forward from the oval part of the 

 body as far as the junction of the cerebropedal and cerebrovisceral connectives. 



In favorable material it is possible to isolate the otocyst sufficiently from its sur- 

 rounding tissues so that it can be observed in a living condition under the mircoscope. 

 To do this it is necessary to select animals whose sex glands are spent or undeveloped 

 and which have been starved for several days. The animals should then be narcotized 

 with a saturated solution of chloretone in sea water or with cocaine to bring about 

 complete relaxation of the muscles. Then it is possible with small, sharp scissors or 

 with a clean scalpel to cut around the area occupied by the otocyst, strip it off with the 

 epithelium, and spread it out in a drop of water on a microscopic slide where it may 

 easily be examined under the microscope. Under these conditions the otocyst appears 

 as an oval body with a long, slender handle. It is a hollow structure, the walls of the 

 oval part of the body being several times thicker than the wall of the canal and form- 

 ing a clear, thick outer zone which stands in sharp contrast with the dark inner zone, in 

 which vibrating cilia may be seen distinctly (fig. 173, p. 180). The walls of the canal 

 are comparatively thin, and within its lumen the effects of active ciliary movement are 

 visible, the effective stroke of the cilia being inward. 



The visual organs of the sea mussel are of two types, consisting of a pair of well- 

 developed direction eyes and of pigmented epithelial cells of the mantle edge capable 

 of responding to changes in light intensity. Loven (1848) first pointed out that Mytilus 

 possessed an eye in the larval stage. Lacaze-Duthiers in his stud ; es failed to note the 

 fact at all, but Wilson (1887) observed that an eye was present in larvae that had reached 

 the four branchial filament stage. After such authors as Balfour, Fischer, and Lang had 

 stated that the eyes were lost before the adult condition was reached, Pelseneer (1899) 

 announced that the eyes persisted in the adult mussel. List (1902) found that in all 

 the Mytilidse studied by him, the larval eye was retained throughout life and that in 

 all the species it occupied the same position. It is always found at the base of the 

 first anterior inner gill filament on its lateral side (fig. 168, E). Ordinarily it is hidden 



