THE EVIDENCE FROM DEVELOPMENT. 



223 



of segmentation of the egg common to all animals. Sooner or later, 

 however, the young bivalve develops a somewhat rounded body 

 (Fig. 142, A) at the upper or head.-extremity of which appears an 

 expanded disc often 

 described as consisting 

 of two distinct lobes or 

 halves richly fringed 

 with the minute vibra- 

 tile processes called 

 cilia, and named the 

 velum (v). In the centre 

 of this velum, an elon- 

 gated tuft of cilia is 

 usually found in addi- 

 tion, the tuft being 

 known as the flagellum 

 (/). Thus provided 

 with its vibratile " hairs," 

 the young bivalve 

 swims freely through 

 the sea, and is thus said 

 to exhibit its "veliger 



TVi f tT f FIG. 141. CUTTLEFISHES. 



Stage. I The upper figure represents an Octopus swimming backwards. 



substance forms on the 



back of the embryo. This becomes the mantle which lines the shell, 



and in fact forms the latter structure ; whilst in due course the internal 



organs are developed, and the young shellfish assumes the likeness of 



the adult The oyster 



and cockle are thus 



seen to pass through 



a veliger stage (Fig. 



142, A), each with its 



ciliated lobes and its 



free-swimming powers, 



through the exercise of 



which the oyster-spat 



may be conveyed to 



great distances from 



its birthplace. As we 



shall presently note, 



the likeness of this wandering embryo to the young of certain lower 



animals is distinctly marked. 



The curious ship-worm, or Teredo (Fig. 143), which was termed 

 by Linnaeus "calamitas navium," and which effects an immense 

 amount of destruction annually on the wood of our piers and 



FIG. 142. DEVELOPMENT OF COCKLE AND SHIP-WORM. 



