Figure 85. — Transverse section of a small vein of the 

 mantle. Note the ab.sence of endothelium and poorly 

 developed elastic layer. Bouin, hematoxylin-eosin. 



Close nerve contact between the muscles and 

 other organs of the mantle is maintained through 

 a fine nerve network which can be made visible 

 by using the gold impregnation method (fig. 87). 

 I have had no success in revealing it with vital 

 stains. 



PERIOSTRACAL GROOVE AND GLAND 



The narrow space between the middle and the 

 outer lobes of the mantle edge, called the perio- 

 stracal groove (fig. 77, per.gr.), is lined with 

 ciliated epithelium which is replaced at the bot- 

 tom of the groove by glandular cells. The inner- 

 most part of the groove is called the periostracal 

 gland (fig. 88), although it would have been more 

 appropriate to refer to it not as a gland but as a 

 secretory epithelial surface (Maximow and Bloom, 

 1930). This surface is covered with a single 

 layer of glandular cells different in appearance 

 and structure from the epithelial cells of the 

 distal part of the groove. Unlike a true gland, 

 it does not form a compact body extending under 

 the surface of the groove and it has no duct. On 

 transverse sections of the mantle edge the gland 

 sometimes appears as a round structure sur- 

 rounded bv connective tissue. Examination of a 



0.3 



IVI i 1 1 i melers 



Figure 86. — Innervation of the tentacles of the middle 

 lobe. Formalin 5 percent, gold impregnation. Whole 

 mount. 



series of sections shows, however, that this appear- 

 ance is caused by the invaginations of the inner 

 surface of the lobe. The periostracal gland is 

 present in all lamellibranclis and was the object 

 of many histological studies (I^eenhardt, 1926; 

 List, 1902; Moynier de Villepoix, 1895; Rassbach, 

 1912; Rawitz, 1888). 



There is a conspicuous difference in the appear- 

 ance of the cells along the two sides of the groove. 

 Those lining the outer lobe (fig. 88, left side) are 

 distended at the distal ends and taper toward the 

 base into slender rootlike processes which, accord- 

 ing to Rawitz (1888) who described tliem in tlie 

 oyster, penetrate the underlying connective tissue. 

 1 was not able to reveal such rootlets in my 

 material. None of these cells bear cilia, although 

 the distal part of the groove, not shown in figure 

 88, is lined with ciliated epithelium. Typical 

 goblet cells containing eosinophile granules, 

 anioebocytes, and round mucus cells are present 

 in the epithelial layer of botli sides of the groove. 



86 



FISH AND WILDLIFE SERVICE 



