252 



LOWER INVERTEBRATES. 



I'lG. 258. — TrocUosphere 

 of oyster; /, foot; m, 

 mouth; s, sliell glaud; 

 t?, velum. 



Fig. 259. — Veliger of Opistlio- 

 brancb; a, anus; /, foot; i, 

 intestine; o, operculum; r, 

 velum. 



of many worms, and especially the rotifers. At this stage, or even earlier, another 

 important feature appears, the shell gland. Tins is at first an invagination on the side 



of the body opposite the mouth, but 



still outside the velar area. The gland 



soon flattens and begins to secrete the 



shell, which at first appears as a single 



delicate plate. Following the trocho- 



sphere comes the stage known as the 



veliger. The velar area is now a flat^ 



tened plate, fringed with cilia, and fre- 

 quently exjjanded into lobes, while the 



rest of the body is greatly enlarged 



in proportion. The foot is also more 

 prominent. With subsequent development the disproportion 

 between the velar area and the rest of the body increases 

 in all except a very few cases, as the pond-snail, Lmncea. 



Shells, more than any other objects of natural history, have played a part as objects 

 of merchandise, and for the rarities, eonchologists have in times past paid the most 

 fabulous prices. The following, copied from Tryon, may prove of interest to those 

 who have not yet caught the fever of shell-collecting : — 



" Scalaria pretiosa, which can now be had for one or two dollars, was worth 1100 

 in 1735, and $200 in 1701. Phashdiella bidiinoides, which also brought $100, can 

 now be purchased at from one to two dollars, or even less. In 1865 a great English 

 collection, that of Dennison, was sold by auction in London, and some extravagant 

 prices realized. Ct/proea guttata hronght ^200 ; Q/prcea priticeps, the eame; Comes 

 ffloria-nniris, also $"200 ; Conus cerviis nearly $90 ; Conus cedonuUi (not a very rare 

 shell), §90 and 1110; Comes omaicus, (also not rare), $60; Voluta /estiva, $80; 

 Oniscia dennisonii, $90 ; Pholadomyia Candida, $65 ; Carinaria vitrea, (m hich Mont- 

 fort stated to be worth $600), brought $50. The very rare Pleicrotoma quoyana 

 brought in London, in, 1872, $125. In 1876 the Roters van Lennep collection was 

 sold, including: Voluta junonia, $50; ^litra helcheri, $40; Spondylus regiits, 136, 

 etc. For this same Spondylus regvus Professor Richard had previously paid several 

 thousand francs, ^"oluta junonia has always been considered a rare species, and 

 dealers have obtained as much as forty pounds sterling for it. . . . Cyprma 

 unihilicata has been sold for thiity pounds, and may now be had for one pound. The 

 Boston Society of Natural History possesses an Argonauta argo, or paper nautilus 

 shell, which is said to have been purchased by the gentleman who presented it to that 

 society, for $500. It is a common species, and the only reason of the greater valua- 

 tion of this specimen is that its diameter is about two or three inches gi-eater than an}' 

 other individual known to naturalists." 



Class I. — ACEPHALA. 



This group of the molluscs has been burdened with a large number of names. 

 Among them we find Conchifera, Endocephala, Lipocephala, Lamellibranchiata, and 

 Pelecypoda, as well as the older, and consequently preferable, designation adopted 

 here. The group will readily be recognized by all under the popular designation of 

 bivalve molluscs. In this more familiar name is embodied one of the most character- 



