SIBOGLINUM CEYLONICUM 301 



[34. Siboglinum ceylonicum sp. nov. Ivanov (Figs. MM133-00133) 



I found a few specimens of this species at the end of January, 1960, whilst 

 investigating the abyssal fauna on board R.V. Vityaz^ in the Indian Ocean, 

 between Ceylon and the Maldive Islands. In a haul taken with a Sigsby trawl 

 and in three dredgings we took 16 fine-ringed tubes each containing the 

 greater part of an animal. Siboglinum ceylonicum is one of the smaller species 

 of the genus — its body is no more than 0-2 mm across. 



The fore-part of the body is very long, with a length to breadth ratio, in 

 most individuals, of 7 or 8 : 1, but in one specimen, which appeared to be 

 immature, the fore-part was only four times as long as broad. A characteristic 

 feature is the lack of any clearly defined groove between the protosoma and 

 mesosoma ; in front of the bridle there is only a pretentacular groove, cutting 

 off the cephalic lobe. The latter is rather large but no broader than the proto- 

 soma. It is longer than broad, it tapers rather rapidly towards the tip and it is 

 somewhat flattened dorso-ventrally. The epidermis of the cephalic lobe 

 consists of tall cells which contain a clear secretion (Fig. MM 133^4). The 

 anterior part of the protosoma, immediately behind the pretentacular groove, 

 has a rather broad continuous glandular girdle, which appears opaque in 

 transmitted light (Fig. ММ133Л, B). The tentacle, attached at the hind 

 edge of this girdle, has a narrow base, but soon broadens out and then for a 

 considerable length it is remarkable for its stoutness, reaching 0-15 mm — 

 hardly less than the diameter of the body of the animal. The surface of the 

 tentacle is covered with numerous coarse wrinkles, indicating that it is strongly 

 contracted (Fig. ММ133Д B). In the distal part of the tentacle, however, 

 where the circular epidermal folds disappear, it is still frequently ОТ mm in 

 diameter. Pinnules (200/* long) are arranged in two rows along the whole 

 length of the tentacle (Fig. MM133C). Under the microscope the two 

 capillaries in each one are clearly visible. There is a well-marked zone of 

 rudimentary pinnules, similar to that in S. silone sp. nov. (see p. 293), at 

 the base of the tentacle. It forms two thick rows of progressively sprouting 

 pinnules. 



In some individuals the protosoma is slightly constricted behind the point 

 of attachment of the tentacle. The bridle is situated a little in front of the 

 middle of the fore-part, and its colourless keels, lying on clearly protuberant 

 cutaneous ridges, are moderately stout and thickened towards the dorsal 

 ends but do not meet. On the ventral sides of the body each keel turns forward 

 in a characteristic manner, the ends taper slightly and then fuse together 

 (Fig. MM 133Л, B). In transmitted light it can be seen that behind the bridle 

 the coelom of the mesosoma contains numerous rather small spherical glands 



