142 



tessellated cells are seen in their advanced stages of growth, closely 

 packed together, and incorporated with the tissue beneath. One of 

 the largest of the cells, with its accompanying cytoblast in the line 

 of advance (a), measured -rrrs mcn m diameter, while a smaller one, 

 in which the cytoblast and its nucleus were equally distinct, did not 

 exceed IO ooo inch. 



Among the aquatic univalves there are some which have these or- 

 gans in a much greater degree of perfection than others, and in none 

 are they displayed in a more beautiful manner than in the periostra- 

 cum of Lymnea stagnalis, one of the commonest of our fresh-water 

 shells. If the periostracum be removed, in the manner before 

 described, by the aid of dilute hydrochloric acid, and examined as a 

 transparent object, with a linear power of 250, we find them in some 

 parts irregularly dispersed over the surfaces, of an oval form, and 

 usually exhibiting a distinct and very curious description of nucleus, 

 as represented at fig. 1, PI. xvii. In some of these oval bodies, the 

 nucleus, beneath a power of 550 linear, appeared like a vesicle which 

 had collapsed irregularly, as represented in fig. 6, PI. xvii. while in 

 others it assumed the appearance of a short, thick, convoluted thread, 

 as represented in fig. 5. This curious form of the nucleus is different 

 from any that I have observed in other shells. In other parts of the 

 same periostracum the cells assumed quite a different form (that of an 

 oblong), and were arranged end to end in short branching lines, as 

 represented in fig. 2, PI. xvii. ; and when view r ed with a power of 

 550 linear, many of them were found to contain numerous minute 

 globular bodies, probably incipient cytoblasts, as seen in fig. 3, PI. 

 xvii. One of the largest of this description of cell [a, fig. 3) measured 

 T 4 t i inch in length by tttt m breadth. The whole of the perios- 

 tracum of this shell, when in its adult state, appears to be covered 

 by an exceedingly fine membrane, thickly studded with minute ma- 

 millated spots (a, fig. 2, PI. xvii.). These spots certainly cannot be 

 considered to be cytoblasts, from the regularity of their disposition, 

 and from the universal absence of a nucleus, even when examined 

 with a linear power of 1020, as represented by fig. 4, PL xvii. One 

 of the largest in this group which I measured was t^^ inch in 

 diameter. 



The periostracum of Trigonia Lamarckii furnishes another ex- 

 ceedingly curious variety of the development of membranous tissues 

 through the agency of cytoblasts. In my observations " On the Or- 

 ganic Tissues in the Bony Structure of the Corallida3," (' Philoso- 

 phical Transactions,' part 2, 1842, p. 220, plate 17, figs. 3 and 4), 



