8. THE TENTACULAR APPARATUS AND EXTERNAL DIGESTION 



83 



Thus in each capillary two parts may be distinguished — the long intra- 

 cellular part inside the pinnule and the short intercellular section linking it to 

 the corresponding tentacular vessel. It is interesting to observe that a single 

 spindle-shaped cell with an elongated nucleus (Fig. 53, cca) is always found 

 lying against the wall of the intercellular section. 



The vessels of the tentacle itself are usually of different diameters because 

 of the unequal content of blood. They lie asymmetrically on opposite sides 

 of the tentacle, the afferent vessel nearer to the pinnular internal side of the 

 tentacle and the efferent opposite, nearer to the epidermal nerve (Fig. 53). 

 The thin intima of the vessels is closely bound to the basement membrane of 

 the epidermis (Fig. 53). 



In its histological structure the whole system of blood 

 vessels of the tentacles presents a series of paradoxical 

 features. Not only is the presence of intracellular vessels 

 surprising but so also is the way in which they form a 

 single unified system with vessels of a more normal type. 

 For a complete understanding of these singular features 

 it is necessary to study the ontogenetic development of 

 the tentacles and pinnules. 



It is clear from what has been written above that the 

 pinnules represent specialized epidermal cells. Their 

 origin, growth and differentiation are not limited to one 

 period in postembryonic development but continue 

 throughout the whole life history of the animal. On 

 each tentacle there is a narrow zone where new pinnules 

 are forming, lying between the short basal part (which 

 always lacks pinnules) and the remaining part, which is 

 furnished with fully formed pinnules. The earliest stages 

 in pinnule formation can be studied in the proximal part 

 of this zone. Distally they become progressively more 

 differentiated until they are fully formed pinnules. A few 

 successive stages in pinnule development are illustrated 

 in Fig. 54. 



-ps 



Fig. 56. Front part of the body of Siboglinum timorense. 

 ms - mesosoma ; mts - metasoma ; pi - pinnule ; ps - protosoma ; / - ten- 

 tacle. (After Caullery, 1944, with modifications.) 



— ms 



{—mts 



