TYPE MOLLUSC A. 283 



to be found forming the osphradia, also supposed to have an 



olfactory function. Otocysts (Fig. 125) are present in nearly 



all the groups, consisting of a vesicle with a membranous wall, 



the interior of which is lined by sensory cells bearing bunches 



of hairs projecting into the vesicle 



which contains one or more calca- 



reous otoliths. An auditory func- 



tion has usually been attributed to 



these organs, but it seems probable 



that, as in the lower forms (see p. 



82), they are rather to be regarded 



as organs of an equilibrium-sense, 



and in fact that they subserve such 



a function in part at least has been 



experimental^ determined in the 



Cephalopods. 



T-, , ,, , FIG. 125.- OTOCYST OF Ptero- 



Eyes are very frequently present 



iafter CLAUS) 



and in the Cephalopods reach an 



exceedingly high development. They occur usually upon the 

 head, but may also be found, as in the Pelecypoda, upon the 

 edge of the mantle, or even on the dorsal surface of the body, 

 as in the Pulmonate Oncliidium. They vary, however, so 

 much in structure in different groups that an account of the 

 various modifications which they present may be postponed. 



Excretory organs in the form of a pair of nephridia are 

 present, each nephridium consisting of a tube which opens at 

 one extremity into the mantle-cavity, while at the other it 

 communicates^ with the cavity of the pericardium, which, as 

 has been seen, is a portion of the coeloru. The relationships 

 of these structures are therefore the same as those of the 

 nephridia of the Annelids, and, as in those forms, they receive 

 a rich supply of blood, most of the venous blood returning from 

 the tissues passing through their walls on its way to the 

 branchiae. The reproductive organs are unpaired in the 

 majority of forms and in some cases come into relation with 

 the nephridia, which serve as reproductive ducts. More 

 usually, however, they open directly to the exterior, a con- 

 dition which is probably a secondary one. The majority of 

 the Mollusca are bisexual, but hermaphroditism is by no 



