Figure 76. — Whole 

 portion of connect 

 Safranin stain, 

 blood vessels; cp 

 cumpallial nerve; 

 tain (inner lobe) ; 

 or shell lobe; ti— 

 of middle lobe, 

 muscles are not 

 toxvlin. 



Cenfimeters 



mount of a piece of mantle. .Major 

 ive tissue was removed by maceration. 

 Magnified about 10 times, bl.v. — 

 a. — circumpallial artery; cp.n. — cir- 

 m.l. — middle lobe: p.c. — pallial cur- 

 r.m. — radial muscle; o.l. — outer lobe 

 -tentacles of inner lobe; t2 — tentacles 

 Radial nerves surrounded by radial 

 visible. Formalin 5 percent, hema- 



and Rai, 1931), "reduplications" (Nelson, 1938; 

 Pelseneer, 1906), "lamellae" (Hopkins, 1933), 

 "lames" in French (Leenhardt, 1926), and 

 "Klappe", in German (Rawitz, 1888). The term 

 "reduplication" is misleading because the lobes 

 are not formed by the duplication of the mantle 

 tissue, being comparable rather to a fringe or 

 flounce at the margin of a soft material. To avoid 

 confusion the term marginal lobes is retained in 

 this text. 



The mantle border of all the species of oysters 

 studied, namely, C. mrginica, C. angulala, C. 



gigas, 0. edulis, and 0. lurida is divided into 

 three projecting lobes, the outer or shell lobe 

 (sh.L), the middle lobe (m.l.), and the inner lobe 

 or pallial curtain (p.c). Hopkins' statement 

 (1933, p. 483) that "The border of the mantle 

 (of C. gigas) divides into two lamellae, each 

 bearing a row of tentacles" is an obvious 

 inaccuracy of description. 



The outer or shell lobe (sh.l.) is narrow and 

 devoid of tentacles. It lies in contact with the 

 margin of the shell and may be seen protruding 

 beyond the edge of the valve diu-ing periods of 

 rapid growth. The middle and the inner lobes 

 each bear a row of sensitive and highly contractile 

 tentacles. 



The inner lobe or pallial curtain (fig. 77, p.c.) 

 is especially broad and turned inward. In de- 

 scribing this structure in scallops Pelseneer named 

 it the "velum" (1906). Although that term has 

 been used by several investigators (Awati and 

 Rai, 1931; Dakin, 1909b) Nelson (1938) pointed 

 out that the term "velum" is better known as the 

 swimming organ of the pelecypod larvae and 

 proposed to call the inner lobes of the mantle 

 the "pallial curtains". This term seems to be 

 appropriate, but is used in this book in the singular 

 since there appears to be no advantage in the 

 plural recommended by Nelson. 



The inner lobe may be projected into the mantle 

 cavity (fig. 74). Depending on the degree of 

 contraction of various sets of muscles the inner 

 lobe assumes different angles in relation to the 

 mantle as a whole. In a fully relaxed mollusk 

 the lobe of each side extends outward in the general 

 plane of the mantle and shell. In a contracted 

 state the lobes on both sides project inward almost 

 at right angles to the surface of the mantle; in 

 this position the mantle borders touch and the 

 tentacles of the two sides interlock, effectively 

 sealing the entrance to the mantle cavity. This 

 function of the inner lobe was first described by 

 Rawitz in 1888 and was redescribed in 1933 by 

 Hopkins. As will be shown later (p. 304) the 

 pallial curtain also plays an important role during 

 the spawning of female oysters. 



The deep furrow between the shell lobe and 

 the middle lobe is called the periostracal groove 

 (fig. 77, per.gr.), tlie name referring to the secre- 

 tion site of organic shell material by glandular 

 cells concentrated in the deepest portion of the 

 groove and collectively known as the periostracal 

 or conchiolin gland (^c.gl). During the shell- 



THE MANTLE 



733-851 O — 64- 



77 



