SEA MUSSEL MYTILUS EDULIS. 199 



(1887) observed that the cerebral ganglion is the first to be formed in the trochophore 

 larva at the base of the velum in the cerebral pit. Next in order the pedal ganglion is 

 formed. It is readily distinguished as a large group of cells situated at the base of the 

 anterior edge of the developing foot in specimens 0.36 mm. long (fig. 185, PdG, p. 197). 

 The visceral ganglion appears later on the anterior ventral edge of the posterior adductor 

 muscle, where it is readily seen in mussels 0.385 mm. long (fig. 186, VG, p. 197). At 

 the same time a commissural nerve can be seen growing forward from the visceral 

 ganglion. In specimens 0.512 mm. long commissures are completely established between 

 the visceral and pedal ganglia and between the cerebral and pedal ganglia (fig. 1S7, p. 

 1 98) . The direct connection between the visceral and pedal ganglia is soon lost, how- 

 ever, for the commissural nerve grows forward to terminate in the mid region of the 

 cerebropedal commissure which is the adult condition (fig. 163, p. 175). 



SENSE ORGANS. 



A pair of direction eyes appears at the time the first gill papilla? are formed. The 

 position they occupy corresponds to what will be the base of the first anterior inner gill 

 filament. They are formed from a cuplike invagination of ectodermal cells which later 

 become filled with a mass of dark brown granules that make the eyes appear as conspic- 

 uous dark round spots in specimens rendered transparent (figs. 185 and 1S6, E, p. 197). 



Shortly after the eyes are developed a pair of otocysts arise as ectodermal invagina- 

 tions just dorsal to the eyes. The invagination proceeds backward to a position where 

 the capsules with their contained otoliths lie dorsal to the pedal ganglion (fig. 186, Ot). 

 As growth of the animal continues the pedal ganglion shifts backward, leaving the otocyst 

 inclosed in the angle formed by the union of the cerebropedal and cerebrovisceral 

 commissures (fig. 163, Ot, p. 175). 



The osphradium and the abdominal and pallial sense organs are clearly modifica- 

 tions of the body epithelium, but at just what period the transition takes place was not 

 determined. 



PERICARDIUM. 



The pericardium is first seen when the young mussel reaches a length of 0.3S5 mm. 

 It arises from a mass of mesodermal cells around the terminal intestine just dorsal to the 

 posterior adductor muscle (fig. 1 86, P, p. 1 97) . As the intestine lengthens, the pericardium 

 migrates forward to the mid-dorsal region of the body (figs. 187 and 188, P, p. 198). 

 Pulsations of the heart were noted by Wilson (1887) as first visible in embryos 0.65 mm. 

 long, possessing 10 or 11 gill papilla?. 



GENITAL ORGANS. 



The genital organs begin to form after the animal has almost completed its metamor- 

 phosis. According to the author's assumption, based on the observations of Ziegler (1885) 

 on Cyclas cornea, the paired mass of mesodermal cells, which appears just dorsal and ante- 

 rior to the posterior adductor muscle, in close relation to the pericardium when the 

 embryo is 0.385 mm. long, is the forerunner of the reproductive system (fig. 186, G, p. 197). 

 This position corresponds to the external opening of the genital system in the adult. At 

 this stage the further course of development from the cell group was lost . In the next 

 stage of the author's series, which was 0.72 mm. long, no trace of the cell groups could 



