inner and middle lobes was found to vary from 

 10:18 to 10:32. 



It has not yet been definitely established 

 whether the two types of tentacles contain different 

 receptors and therefore respond to different 

 stimuli. According to Elsey (1935) the large 

 tentacles of C. gigas are more sensitive to hydro- 

 chloric acid than the small ones. Hopkins (1932) 

 does not specify which row of tentacles was under 

 (ihservation in his work on sensory stimulation 

 of C. virginica. In my experiments (see p. 293) 

 dbservations were made exclusively on the long 

 tentacles of the inner lobe. 



A narrow and slightly pigmented cylindrical 

 structure along the dorsal edge of the mantle 

 (fig. 78) marks the position of the subligamental 

 ridge, the organ which secretes the ligament. 

 The ridge consists of a layer of specialized epi- 

 thelium underlined by connective tissue. Large 

 blood vessels are found close to the base of the 

 ridge. Microscopic structure of the ridge is given 

 on p. 83. 



RUDIMENTARY MUSCLE OF THE 

 MANTLE 



A small and sometimes hardly visible muscle is 

 located on the dorsal part of the mantle. Its 

 location is sometimes marked by light violet 

 pigmentation and by a shallow depression in the 

 corresponding part of the valve to which the 

 muscle adheres. The attacliment is weak, and in 

 the majority of oysters the muscle Separates from 

 the valve when the valve is lifted. Leenhardt 

 (1926) states, however, that in some 0. ednUs the 

 muscles were so strongl,y attached to the shell that 

 they could not be separated without rupturing the 

 mantle tissue. Examination of sections of the 

 mantle of C. virginica from the Woods Hole area 

 convinced me that muscle fibers do not extend 

 from one side to the other, but end in the con- 

 nective tissue of the mantle. Th& muscle is 

 apparently nonfunctional and morphologically is 

 not analogous to the anterior adductor of bivalves. 

 Leenhardt (1926) considers the rudimentary 

 muscle of the mantle as a vestige of the larval foot 

 retractor which disappears during metamorphosis. 

 Stenzel (1963) states that this musde is present in 

 all the Ostreidae and calls it Quenstedt's muscle in 

 honor of its discoverer (Quenstedt, 1867). 



HISTOLOGY 



The mantle consists of connective tissue which 

 envelops the muscles, blood vessels, and nerves 



and is covered on both sides with the epithelium. 

 CONNECTIVE TISSUE 



The most conspicuous structural element of the 

 connective tissue is the vesicular cell, characterized 

 by large globular or oval body and relatively small 

 nucleus without nucleoli. In zoological literature 

 these cells appear under a variety of names and 

 were even incorrectly considered as lacunae 

 (Leenhardt, 1926) and mucus cells (List, 1902). 

 Well-developed membranes outline cell boundaries 

 sharply; the protoplasm within forms a delicate 

 network of fine granules. In preparations dehy- 

 drated with alcohol the inside of the vesicular 

 cells appears almost empty, but in tissues treated 

 with osmic acid and in frozen sections stained 

 with Sudan II and other fat stains large globules 

 of lipids are seen to fill the inside of the cells (figs. 

 79 and 80). Less abundant are the smaller round 

 cells with more compact protoplasm. They often 

 occur near small arteries (fig. 81, r.c). The 

 fusiform cells (f.c.) with small bodies and oval 

 nuclei form long branching processes which anasto- 

 mose and touch each other. 



Examination of frozen sections of connective 

 tissue treated with toluidine blue or other meta- 

 chromatic stains shows clearly the presence of a 

 cytoplasmic ground substance with a very fine 

 reticulum supporting various inclusions. After 

 the removal of glycogen this substance can be 

 stained very deeply with periodic acid fuchsin 

 (McMannus reagent )or with Hale stain which is 

 used to test for acid polysaccharides of the hyalu- 

 ronic acid type (Hale, 1946). The results of such 

 staining reactions have been interpreted in the 

 literature as indicating the presence of mucopoly- 

 saccharides or mucoproteins. Histological meth- 

 ods are not entirely dependable (Meyer, 1957), 

 but so far no chemical analyses of the connective 

 tissue of the mantle have been made. It is known, 

 however, that acid mucopolysaccharides are 

 among the components of the ground substances 

 in mammalian tissues. It is very likely tnat they 

 are also present in the connective tissue of the 

 oyster. 



Elastic fibrils are scattered throughout the 

 connective tissue of the entire thickness of the 

 mantle but appear to be more abundant at the free 

 edge and in the layers underlying the surface 

 epithelium (fig. 77, el.f.). Muscle fibers are also 

 very abundant and will be discussed in detail later. 



In some specimens the mantle may be thin and 

 transparent whereas in others it is thick and 



THE MANTLE 



79 



