2 3 o CHAPTERS ON EVOLUTION. 



arisen, if the story told by development is worthy of credit, from 

 a common root-stock, of which the " veliger-stage " is the transient 

 representative. Both have developed in parallel, or, it may be, in 

 corresponding and similar grooves. But the gasteropod has been 

 evolved beyond its " veliger-stage " to assume a higher place in the 

 animal series ; whilst the pteropod has been arrested in its development 

 at this stage, and has assumed, with possibly a little fixation of its cha- 

 racters, a larval condition as the badge and mark of adult structure. 



Passing, last of all, to a lower group of molluscs that of the 

 BratkwpodOi or "Lampshells" (Fig. 154, A) we may find through 

 these latter forms a passage to the still lower and more primitive stock 

 from which the Molluscan group may be presumed to have originated. 

 The brachiopods form a scarce group of shellfish in our present seas, 

 but in past epochs of this world's history they were abundantly 

 represented The Silurian rocks, to mention but one group of 

 formations, literally teem with their fossil representatives, whilst the 

 paucity of these shells in existing waters is matter of zoological 

 notoriety. These " Lampshells " are, therefore, an extremely ancient 

 group of living beings. That they are inferior in many phases of 

 structure to the common bivalves such as our oysters and cockles 

 is matter of fact. Hence the development of these " Lampshells " may 

 be presumed, on a priori grounds, to be fraught with meaning and 

 information as to the descent and origin of the Mollusca at large. 

 Let us, therefore, endeavour to follow out the researches of Morse on 

 the development of these singularly interesting forms. 



Studying one species Terebratulina, the common " Lampshell " 

 of the American coast the first free-swimming stage is that of an elon- 

 gated body (Fig. 154, B), which divides itself crosswise into three rings 

 or segments (C, D), the front one of which becomes provided with long 

 actively-moving cilia. Eye-spots also appear on the front segment, and 

 the likeness of the young lampshell to an embryo-worm (Fig. 157, C) is 

 at this stage plainly apparent. Nor is the likeness lessened, when the 

 middle segment is found to develop four bundles of seta, or bristles, 

 such as appear in the worms. Then succeeds the stage of fixation. 

 The young brachiopod now attaches itself by its lower segment (E), 

 and the middle segment increases greatly in size, so as to form a kind 

 of hood enclosing the front segment in part. Then the front segment 

 (F, G) decreases in size; the middle portion originates the bivalve shell 

 (H, V, d), which soon comes to enclose the body (c\ the lower or third 

 segment being represented by the disc or stalk of attachment (/). 

 The technicality of the subject prevents our following out for the 

 reader the later stages of lampshell growth, in which striking likenesses 

 are presented, not merely to brachiopods now extinct, but likewise to 

 the young stage (Fig. 154, I) of those plant-like animals, named 

 Po/yzoa, and of which the Sea-mats (Flustra) (Fig. 190, a) of our 



