110 SEXUAL ORGANS. 



• sargassums, suspended by one or several filaments. If these cords 

 be broken, the mollnak emits an air-bubble . enveloped by a 

 glutinous substance which rises to the surface of the water, 

 carrying with it the end of a new filament : upon coming in 

 contact with the fucus a new attachment is formed. 



The lanthina constructs a floating apparatus or raft (xvii, 99), 

 attached to its foot and comprised of a quantity of airy vesicles. 

 Lacaze-Duthiers states that upon detacliing the float the animal 

 sinks ; but that the groove of the foot imprisons some air-bubbles 

 under a glutinous envelope, by the aid of which it again seeks 

 and maintains itself upon the surface of the water until a new 

 raft is developed. 



Many of the lamellibranchiates secrete a byssus. Its gland 

 is situated in a groove towards the base of the foot, on its ventral 

 surface. It produ.ces a number of elastic, adhesive filaments 

 by tlie aid of which the animal attaches itself to submarine 

 bodies (iii, 41). In Tridacna, Avicula and Area the filaments 

 cohere, forming a homogeneous mass ; in Pinna the fibres are 

 distinct, fine, numerous, silky, and capable of being woven, so 

 that gloves have been made from this material. In Mytilus the 

 filaments are frequentl}' separated at the attached end so as to 

 occupy several points of anchorage (iii, 40). 



Many embryonic Unionidse are known to possess a byssus, and 

 a few adult species also have it ; in Unio acutissimus it is over 

 an inch in length (Lea). By means of its byssus a bivalve 

 mollusk is able to ascend perpendicular surfaces. It abandons 

 at will the various threads it has woven, and also breaks those 

 which impede its movements. The activity of the bj^ssal gland 

 of Pecten varius is such that Fischer has observed at Arcachon, 

 an individual secrete sixty byssal bu.ndles in eight da3^s, during 

 which time it had elevated itself on the aquarium glass to a 

 height of about two feet — each abandoned byssus being attached 

 about -4 inch below its successor. 



The bj^ssal plug of Anomia is the calcareous product of a gland 

 replacing the byssal gland ; morphologically it is a calcified 

 byssus. 



SEXUAL ORGANS. 



There is much diversity in the reproductive organs of mollusks, 

 and in some of them the whole history of reproduction is fraught 

 with so much interest that I deem it advisable to devote a few 

 Images to the subject. The reproduction of mollusks is in all 

 cases eflfected by means of eggs. In a few gastropods the young- 

 are hatched and retained for a period within the oviducts of the 

 parent — such ovoviviparous species are Paludina, Cj^mba, etc.; 

 the eggs of the acephala also sometimes remain within the parent 

 shell until hatched. 



