318 ANIMAL BIOLOGY. [Part II. 



anodon. The general surface is sensitive to touch, but no special 

 end organs have been described. The labial palps have a rich 

 nerve-supply ; we may conjecture that they minister to taste 

 we cannot at present do more. The papillary processes of the 

 inhalent siphon are sensory, but to what sense they minister 

 we are uncertain. The thin layer of elongated epithelial cells in 

 connection with the olfactory ganglion is held to be an organ of 

 smell (osphradium) and the olfactory ganglion is regarded as a 

 centre for the co-ordination of sensations received in connection 

 with the inhalent current. The auditory organ is a small sac 

 lying posterior to the pedal ganglion of each side. It is difficult 

 to dissect out in the mussel. But in the small fresh-water 

 mollusc Cyclas cornece it may without difficulty be seen if the 

 transparent foot be mounted for the microscope and examined 

 under a low power. It is a small sac, containing a mobile 

 particle or otolith. The sense of sight is apparently absent in the 

 mussel. 



The Reproductive Organs. The mussel is dioecious. The 

 generative opening (Fig. 92, g. o.) is just anterior to the external 

 opening of the renal organ. It opens into a duct which gives off 

 a great number of csecal branches. The reproductive organs are 

 thus similar in the two sexes, and occupy in winter and spring 

 much space in the upper part of the foot Unless a little of the 

 organ be examined microscopically it is not easy to distinguish 

 the testis from the ovary, 



If a little of the testis be teased in normal saline vast numbers 

 of spermatozoa will (in winter) be seen dancing in the field of the 

 microscope, especially if the slide be warmed. They have short 

 rounded heads, and long vibratile tails. 



If a little of the ovary be teased, ova in various stages of de- 

 velopment will be seen. Fig. 94, 1-3, shows such ova. Externally 

 is a vitelline membrane, produced in immature ova into a spout- 

 like tube, through the aperture of which (mwropyle) the proto- 

 plasm of the ovum was originally in connection with the ovary. 

 In developing ova the protoplasm is granular, and stains readily. 

 But an ovum that has been dehisced, or set free, is wonderfully 



