AFFINITIES OF ECHIURUS UNICINCTUS. 95 



diameter of sjeneric importance ; (3) the presence of a very definitely marked belt of 

 compound 2,'lands in the skin is a feature peculiar to this species, (4) as is also tlie single 

 circlet of peri-anal hooks, and (5) the spiral arms of the inner funnels of the segmental 

 organs ; (6) there is also no special mesentery nor diaphragm such as occur in Echiurus 

 Palhisii and Tlialassema neptuni ; and the alimentary canal seems to be simpler, for I 

 cannot distinguish the various regions named by Spengel, Jameson, and others. 



'lliese peculiar features seem to me to have more than specific significance, and may 

 make it advisable to place this Japanese worm in a separate new geuus. On making 

 dissections of Ecliiiiriis laiicinctuH and Fallasli, and Thalassema neptuni, the identity 

 between the two latter was very striking, the only fundamental difference being that 

 Thalnsscnta lias no peri-anal bi'istles, its blood-system, prolioscis, mesenteries, nephridia, 

 body-wall, and alimentary canal all showing close agreement with Echiunis Pallasii ; 

 whereas, in these numerous and important respects, the Japanese " species ' is utterly 

 unlike the British Echiurid. Drasche, however, says " dieser neue Echiurus verbindet 

 also Merkmale von Thalassema mit deueu von Echvurus und erweitert somit nicht allein 

 die bisherigen Anschauungen iiber der Bau der Segmentalorgane l)ei Echitirm, souderu 

 moditieirt auch durch seinen nur einfaclien Hakenkranz die Umgrenzung der Gattung 

 selbst, wie sie bislier augenommen werden." His examination of tiie species, however, 

 was limited to two specimens, and was necessarily not exhaustive ; but after the facts 

 which have now come to light it would be necessary to extend the limits of the genus 

 very considerably, if the species unicinctus is to be retained within it. 



EXPLANATION OF THE PLATES. 

 Plate 7. 



Fig. 1. Echiurus unicinctus (natural size). 



2. Ventral view of the anterior extremity (enlarged) . 



3. Anterior seta, side view. 



4. Piece of the external surface (magnified), showing the papilla! elongated transversely. 



I A = anterior, j P = posterior. 



5. Lateral dissection ; alimentary canal displaced. 



6. Longitudinal section through the body-w;ill in the region of the segmental organ.s. 



7. Section through a compound gland (highly magnified). 



8. Transverse section througli the body-wall in the middle of the worm. 



9. „ „ „ „ at the posterior end of the worm. 



10. Longitndinal muscle-fibres of fig. 9, more highly magnified. 



11. Section through a papilla on the proboscis, showiug a group of sensory cells. 



12. Dissection of the anterior end, showing the absence of blood-vessels aud of the "' diaphragm or 



septum " which occurs in E. Pallasii- 



