DEVELOPMENT OF ANODONTA 455 



The egg-cell is surrounded by a vitelline membrane, and attached to 

 the wall of the ovary by a minute stalk, the insertion of which is marked 

 on the liberated ovum by an aperture or micropyle, through which the 

 spermatozoon enters. 



Segmentation is total but unequal. A number of small clear yolkless 

 cells are rapidly divided off from a large yolk-containing portion, which 

 is slower in dividing. Eventually a hollow ball of cells or blastosphere 

 results. 



On the posterior dorsal region a number of large opaque cells form 

 an internally convex plate — the beginning of the future shell-sac. A 

 pah: of large cells are intruded into the central cavity, and begin the 

 mesoderm. 



On the under-surface posteriorly there is a slight protrusion of ciliated 

 cells forming a cihated disc. In front of this, at an unusually late stage, 

 an invagination estabhshes the archenteron, and the embryo becomes a 



gastrula. 



The shell-sac forms an embryonic shell, and many of the mesoderm 

 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 (see Fig. 257). 



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 manages 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 true byssus gland 

 (which remains quite rudimentary in Anodonta) 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 Hable to be s^wept 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 



