84 ALICE L. EMBLETON ON THE STEUCTUKE AND 



tlie ventral vessel and encircles the intestine, opening into the posterior end of the 

 dorsal vessel. In Echiurus this same vessel encircles a stout muscle which runs from 

 the base of one of the ventral bristles to the other." 



On examining Echiurus unicinctus, however, no trace of a closed system of blood- 

 vessels could be found. Many individuals were very carefully opened, but always with 

 the same negative result, there being no dorsal nor ventral vessel with their connecting 

 loop or " muscle-ring" (P). 7. tigs. 5, 12). Microscopic jiveparations served but to verify 

 this oliservation ; no vessel exists in relation to the nerve-cord, as is seen in sections 

 of the entire worm and in those of the nerve-cord alone (PI. 9. figs. 22, 23, 24, 25). 

 If the loop connecting the dorsal and ventral vessels were present, encircling as it does 

 in other species the muscle running between the basal ends of the two anterior hooks, it 

 w^ould ajjpear in the section of the. entire worm in that region (PI. 9. fig. 22, m.b.), 

 drawn with the " camera lucida," In similar sections taken through specimens of 

 Echmriis Pallasii and Thalassema nejituni the blood-vessels were unmistakably present, 

 cut across at various angles. Fearing my methods might be at fault, I applied them 

 to preserved material of Echiurus Pallasii and Thalassema neptuni ; both in dissections 

 and microscopic preparations the vessels occur as Spengel and Jameson describe, being 

 quite clear and distinct. In sections of tlie proboscis in these two allied forms, the 

 median and latei-al vessels are seen at once ; but sections in this region of Echiurus 

 nnicinctus are very ditFerent : there are no definite vessels present, but irregular sinuses, 

 which appear to be in two series — conceivably an efferent and an afferent system. 

 The ventral sinuses are elongated transversely (PI. 9. fig. 26, s.v.), and are rather 

 undefined, with thin walls w^hich are scarcely discernible. The upper sinuses, elongated 

 in the opposite direction, are less irregular, with more definite walls (tig. 26, s.d.). 

 In tracing the series of sections with great care from the anterior extremity of tlie 

 proboscis to its union with the body, it is seen that gradually the sinuses widen out, 

 forming large cavities (PL 9. fig. 24), which eventually run together in the mouth-region 

 and then merge into the body-cavity. Ova and coelomic corpuscles are found in all these 

 sinuses {o,c.), even at the tip of the j)roboscis. This is conclusive evidence, establishing 

 the fact that these sinuses are but forward extensions oi the coelom. 



These sinuses are in all probability largely connected with the protrusion of the 

 proboscis, functioning much the same way as the blood-sinuses in the foot of the 

 Lamellibranchia, where the muscular foot is protruded by an injection of blood into its 

 substance. 



Drasche, in speaking of this species, says : " Trotz sorgfiiltiger Untersuchung beider 

 Exemplare gelang es mir leider nicht, die Gefiissschiiuge aufzufiuden." 



The fact that Drasche failed to detect the vascular loop in his tw'o specimens sujiports 

 my observations, which have the advantage of having been made on many individuals. 



Nervous System. — The nervous system is arranged in a manner essentially characteristic 

 of all Ecliiurids. There is a single ventral nerve-cord, extending from mouth to anus 

 (PL 7. figs. 5, 12, n.) ; it is fixed to the ventral surface of the body-w^all by its lateral 

 branches, which for a short distance are quite free, then they peneti'ate into the tissue of 

 the body-wall, running parallel Avith the circular muscles (Pi. 9. fig. 27). These lateral 



