248 PART II. SYSTEMATIC ACCOUNT 



Material : five tubes, of which three contained animals. 



Locality: the Pacific Ocean to the east of the North Island of New Zealand. 



Depth: 3013 m. 



Substratum: silty clay. 



20. Siboglinum minutum Ivanov (Figs. 4, 124-5) 



Ivanov, 1957a: 449, 451-3, 458, 459, 491, 496, 497, Figs. 20-22, 59; Ivanov, 1958a: 

 1369; Ivanov, 1960a: 1541, 1542, 1545, 1550, 1587, 1606, Figs. 1386, 1405; Ivanov, 1960c: 

 5, 10, 19, 27, 35, 75, 91, 92, 108, 126, 158, 167, 180-3, 197, 266, Figs. 4, 87, 124-5. 



This species, encountered in abundance in a few places in the abyssal zone 

 of the Bering Sea, is one of the very smallest representatives of the genus 

 Siboglinum. Its hair-like, very slender tube is distinguished by its charac- 

 teristic appearance and its red-brown colour. The small diameter of the tube, 

 the hardness and brittleness of its walls and the softness of the animal itself, 

 all contribute to make it almost impossible to dissect it out. For this reason, 

 despite the abundance of the material available, not all the parts of the animal 

 have been extracted from the tube. The anterior section of the body, a little 

 flattened dorso-ventrally, is four and a half to five times as long as broad 

 (Fig. 124у4, В). The small size of the protosoma and its peculiar shape are 

 very characteristic, and the annular groove, limiting it behind, runs imme- 

 diately behind the base of the tentacle. The mesosoma is five or six times as 

 long as the protosoma, if the cephalic lobe is not taken into account. The 

 protosoma, moreover, is made up of three parts : a triangular central part, 

 including the base of the tentacle, and two swollen lateral parts, which are 

 particularly sharply delimited on the ventral side of the body (Fig. 4, 124Л, 

 B). The conical cephalic lobe is like-wise very small, and because of the 

 peculiar shape of the protosoma it is difficult to determine how far it is cut 

 off from the latter. The long slender tentacle apparently has no pinnules, but 

 this has not been determined for sure, because of inadequate fixation of the 

 material. The keels of the bridle, which is situated a little in front of the 

 middle of the mesosoma, lie on low cutaneous ridges, and are fused ventrally 

 and almost meet dorsally. Under the miscroscope it is evident that each con- 

 sists of a delicate cuticular thread, along which is arranged a series of very 

 delicate transverse parallel rodlets, whose axes lie at right angles to the thread 



Fig. 124. Siboglinum minutum: A - front end of a female in ventral, and, В - in dorsal view; 

 С - keel of bridle under high magnification ; D - a portion of the non-metameric part of the pre- 

 annular region; E - region of thickened papillae; F - region of the girdles; G - part of a girdle; 

 H - part of the postannular region. 

 an - girdles; с - cephalic lobe; со - dorsal ciliated band; dp - hind group of teeth;/- keel of the 



