118 PART I. GENERAL ACCOUNT 



in Oligobrachia dogieli, which has six to eight or even nine tentacles in 

 the adult. As we have already seen (p. 27) new tentacles continue to be 

 formed even in the sexually mature state and their formation is concentrated 

 at the ends of the horse-shoe base of the tentacular crown. It is likely, there- 

 fore, that the same thing occurs in ontogeny. Evidently the process of 

 successive formation and growth of tentacles, first begun in the embryo, 

 continues throughout life and possibly ceases only in old age. The right 

 anterior tentacle is the first to be formed, then its pair on the left side. After 

 that they probably appear right and left alternately, thus maintaining the 

 symmetry of the crown. 



The question of the nature of the single unpaired tentacle of Siboglinum 

 arises in this connexion. We have already seen that it is homologous with 

 first tentacle of Oligobrachia. It is possible to speculate that it is the primitive 

 condition in Pogonophora to produce only the one tentacle and that all the 

 rest are a secondary development. Viewed in this light Siboglinum would be 

 the most primitive genus and the origin of a single tentacle during ontogeny 

 might be looked upon as recapitulation. But this supposition runs counter to 

 the observation that the tentacle of Siboglinum is innervated from the right 

 side of the brain (p. 26 and Fig. 38) and this same nerve runs asymmetrically 

 up the right side of the tentacle. It is difficult to imagine that such asym- 

 metry is primitive in an animal which in all other respects shows a strong 

 bilateral symmetry. It is easier to postulate a secondary reduction of the 

 number of tentacles. In Siboglinum, then, only the right anterior tentacle 

 remains out of the whole tentacular crown. It is apparent that the reduction 

 of the rest of the tentacles was accomplished by a failure of the later stages of 

 development of the crown, i.e. in the terminology of A. N. Severtsov (1939), 

 through "negative anaboly". Thus the presence of only a single tentacle in 

 Siboglinum is the result of secondary reduction of the greater part of the crown, 

 and at the same time represents an embryonic feature carried on into adult life. 



The archenteron is probably formed by delamination within a compact 

 mass of cells, from which the coelomic primordia are later divided off. An 

 epithelial gut rudiment begins to form after a great delay but a blastopore and 

 a secondary mouth are completely lacking. All these features clearly show the 

 pronounced secondary changes in the embryonic processes and are undoub- 

 tedly associated with the absence of a gut in the adult. 



Nothing is yet known about the ultimate fate of the gut rudiment, a 

 question of the greatest importance. To judge from the organization of the 

 adult, in which no trace of gut remains, the gut primordium serves only as 

 the main food reserve of yolk and oil, and soon after the young animal takes 



