292 INVERTEBRATE MORPHOLOGY. 



A mucli more peculiar series of organs, found, however, in 

 their perfect form only in certain species, is developed in 

 connection with the shell of the Chitouidse. They consist of 

 club-shaped structures contained in pores which traverse the 

 shell-plates and possess a definite arrangement, being ar- 

 ranged in groups of larger and smaller organs (meg alee stlietes 

 and micrcesthetev). Each group is in connection Avith a num- 

 ber of large glandlike cells, which terminate in the megal- 

 sesthete, covered externally by a cup-shaped layer of chitin, 

 and from this cell-mass more or less numerous branches 

 arise, the micrsesthetes, which terminate in club-shaped 

 swellings likewise covered by a chitinous layer. Below the 

 group of cells is in connection with fibrils which unite to 

 form a nerve probably passing to the pleuro-visceral nerve- 

 cords, and it thus seems tolerably certain that these struc- 

 tures are sensory and perhaps tactile in function. In some 

 species the megalaesthetes become modified into eyes consist- 

 ing of an external convex chitiuous cap, the cornea, below 

 which is a lens and below this a layer of retina-cells con- 

 nected with nerve-fibrils and surrounded by a cup of pig- 

 ment-cells. No eyes other than these occur in the Polypla- 

 cophora, nor are tactile tentacles or otocysts, of such fre- 

 quent occurrence in other Mollusca, found. 



The nephridia (Fig. 130, n) are paired, one lying on each 

 side of the body and consisting of a long tube giving rise to 

 numerous dendritic branches. Posteriorly the tube branches, 

 one of the branches opening into the mantle-cavity in its 

 posterior part, while the other communicates with the peri- 

 cardial portion of the enteroccel. In function these organs 

 of the Chitouidse differs from the corresponding ones of the 

 Soleuogastres in being excretory only and in not serving 

 as ducts for the reproductive elements. These are developed 

 in a portion of the euterocoel which lies anteriorly to the 

 pericardium and make their way to the mantle-cavity and so 

 to the exterior by special ducts arising one on each side from 

 near the posterior part of the reproductive euterocoel and 

 ending (go] 011 the sides of the body slightly in front of the 

 openings of the nephridia (no). The Polyplacophora are with- 

 out exception bisexual. 



