224 



CHAPTERS ON EVOLUTION. 



FIG. 143. TEREDO, OR SHIP-WORM, 

 Showing the shell detached. 



harbours, is in reality a bivalve mollusc. Its body is shortened and 

 its breathing-tubes are extended to form the worm-like body, whilst 

 its shells (Fig. 143) are rudimentary and serve as boring-organs. 

 The teredo first undergoes segmentation within the egg (Fig. 142, B), 

 and then appears as an active free-swimming " veliger" (Fig. 142, C), 



differing from the young 

 cockle only in that there 

 is no lash-like "flagellum." 

 Then its mantle and shell 

 are formed, and when five 

 and a half days old, the 

 shells have well-nigh in- 

 vested the whole body. 

 Next the "foot" (/) of 

 the ship-worm is de- 

 veloped, and the velum 

 becomes a crown of cilia 

 (D v\ Then, as the young animal seeks the wooden pile wherein it 

 is destined to bore and ensconce itself, the shells come into play as 

 excavating organs, and, with the growth of the elongated body, ship- 

 worm development may be said to conclude. Thus we find that the 



course of bivalve development is 

 distinctly enough marked. Only in 

 one or two cases (such as that of 

 the fresh-water mussels, Unto) is the 

 " veliger-stage " suppressed. But this 

 latter fact will cause no surprise to 

 the student of development, who is 

 well aware that the effects of varying 

 conditions on the developing young 

 are seen in the production of many 

 changes in an early life-history, and in 

 rendering obscure many phases in the 

 panorama of individual evolution. 



Coming next to the Gasteropods, of 

 which the limpets, whelks, snails, slugs 

 (Figs. 137, 138), and the univalve shell- 

 fish at large, are examples, we find a 

 striking similarity in their early history 

 to the development just sketched. A 



mussel, or oyster, or other bivalve has, as every one knows, no 

 distinct head. This may be the result of degradation. But in 

 the snails, whelks, and their neighbours, the head is plainly enough 

 marked, although in certain low forms of the Gasteropod-class 

 this head-development may not be at all prominent. Such lower 



FIG. 144. DENTALIUM AND ITS 

 STRUCTURE. 



