CHAPTER VI 



THE LABIAL PALPS 



Page 

 111 

 111 

 114 

 118 



Anatomy.-- 



Histology " 



Direction of ciliary currents 



Reaction to stimuli 



1 1Q 



Bibliography 



ANATOMY 



The four soft flaps which lie at. the anterodorsal 

 side of the body under the mantle hood are the 

 labial palps. Their trianoular members are at- 

 tached by their broad bases to tiie visceral mass 

 and have slightly curved margins which extend 

 ventrally to the point where they touch the free 

 edges of the gills (fig. 104). 



The two pairs of palps, one on each side, are 

 joined together into a single unit which serves 

 primarily for final sorting of food particles and for 

 the delivery of food to the mouth. Each pair 

 consists of one external and one internal palp 

 (r.i.p. ; r.o.p.) . The two external palps join together 

 above the mouth (m.) where they form the upper 

 lip (u.l.) ; the two internal palps are united below 

 the mouth into a lower lip (1.1.). As a result of 

 this arrangement the mouth is an irregularly 

 shaped, narrow, curved slit. Both lips are arciied ; 

 the lower one is shorter, and its edge is thicker than 

 that of the upper lip. 



At the central junction of the two internal palps 

 there is a median gutter which leads to the center 

 of the lower lip. The two lateral gutters (l.g.) 

 formed on each side where the external palp 

 meets its opposing internal member are the prin- 

 cipal paths by which the food is conveyed to the 

 corners of the mouth (fig. 104). The surface is 

 smooth along the outer part of the external palps 

 and on the inner palps along the n\edian plane, 

 where the palps meet; along the lateral gutters, 

 both have a striated appearance due to numerous 

 ridges and grooves which empty into the corre- 

 sponding gutter. This arrangement is significant 

 for an effective sorting of food. 

 HISTOLOGY 

 Each labial palp consists of a layer of connective 

 tissue covered on both sides by columnar ciliated 



FISHERY bulletin: VOLUME 64, CHAPTER VI 



epithelium set on a basement membrane. These 

 are made of large vesicular cells of the type found 

 in the mantle (fig. 105, c.c). Within the body 

 of a palp there are many blood vessels and blood 

 spaces or sinuses. These spaces are splits or 

 interstices between the connective tissue cells and 

 have no lining or wall. The longitudinal and 

 transverse muscle fibers are numerous l;)ut not as 

 well developed as they are in the mantle. 



The smooth sides of the palps are covered mth 

 small epithehal cells, about 5 to 6m long, with 

 minute cilia not exceeding .3m in length. Awati 

 and Rai (1931) maintain that in O. cucullata only 

 some of the epithelial cells of this surface are 

 ciliated. They suggest that tlie ciliated cells have 

 sensory function but present no evidence to 

 support this view. The absence of cilia on some 

 of the cells of this layer may be due to then- 

 destruction during the processing of tissue. 



In C. virginica the epitlielium of the smooth 

 surface of the palp consists of almost cubical cells 

 with relatively large nuclei and small cilia (fig. 105, 

 c.ep.). Cell boundaries are indistinct, the cells 

 tliemselves are crowded and compressed, and there 

 is a very thin and transparent cuticle on the 

 periphery. In the subepithelial layer large eosino- 

 philic cells (e.c.) and mucous cells (m.c.) are very 

 abundant. The mucous cells are frequently pear- 

 shaped, but their appearance varies, depending 

 on the amount of secretion they contain. Then- 

 length is between 25 and 30 m- Wherever the 

 secretion of mucus is least, the outer surface of the 

 palp is slightly ruffled, as shown on the right side 

 of figure 106,' representing a longitudinal sectu)n 

 of the palp viewed at low magnification. 



The inner or ridged surfaces of the palps present 

 a dift'erent picture. The entire surface is folded 

 into deep ridges and grooves. In figure 106 the 

 I'idges are about 0.3 mm. high along the central 

 axis. The ciliated cells of the ridged surface are 

 slender, cvlindrical, and tightly packed, with 

 small, round nuclei. They form a layer varying 

 from 40 to 60 m in thickness. The difference m 



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