384 PHYL UM MOLL USCA. 



cells combine in an adductor muscle. The mouth of the gastrula closes, 

 and a definite mouth is subsequently formed by an ectodermic invagina- 

 tion. Gradually a larva peculiar to fresh-water mussels, and known as 

 a Glochidium, is built up. 



The Glochidium has two triangular, delicate, and porous shell valves, 

 each with a spiny incurved tooth on its free edge. The valves clap 

 together by the action of the adductor muscle. The mantle lobes are 

 very small, and their margins bear on each side three or four patches of 

 sensory cells. The foot is not yet developed, but from the position 

 which it will afterwards occupy there hang long attaching threads of 

 " byssus," which moor the larva. If it manage to anchor itself on the 

 tail, fins, or gills of a fish, the Glochidium shuts its valves and fixes 

 itself more securely, and is soon surrounded by a pathological growth of 

 its host's skin. 



In this parasitic stage a remarkable metamorphosis occurs. The 

 sensory or tactile patches not unnaturally disappear ; the byssus and the 

 embryonic byssus glands vanish, but a new byssus gland (which remains 

 quite rudimentary in Anodontd) appears ; the single adductor atrophies, 

 and is replaced by two ; the foot and the gills make their appearance ; 

 the embryonic mantle lobes increase greatly, or are replaced by fresh 

 growths ; and the permanent shell begins to be made. 



After this metamorphosis, when the larva has virtually become a 

 miniature adult, no longer so liable to be swept away, it drops from its 

 temporary host to the bottom of the pond or river pool. 



Third Type of MOLLUSCA. The Common Cuttlefish 

 (Sepia officinalis\ one of the Dibranchiate Cephalopods. 



Habits.- -This common cuttlefish is widely distributed, 

 especially in warmer seas like the Mediterranean. Unlike 

 Octopus, which usually lurks passively, Sepia is an active 

 swimmer ; it moves head foremost by working the fins 

 which fringe the body, or it jerks itself energetically back- 

 wards by the outgush of water through the funnel. It 

 likes the light, and is sometimes attracted by lanterns. 

 The beautiful colours change according to external condi- 

 tions and internal emotions ; and a plentiful discharge of 

 ink often covers its retreat from an enemy. Its food 

 includes fish, other molluscs, and crabs. In spring the 

 female attaches her encapsuled eggs to sea- weeds and other 

 objects, and often comes fatally near the shore in so doing. 

 The cuttles are caught for food and bait. The " cuttle 

 bone ' : and the pigment of the ink-bag are sometimes 

 utilised by man. 



External appearance. A large Sepia measures about 

 10 in. in length and 4 to 5 in breadth; the body, fringed 



