986 SUMMARY OF CURRENT RESEARCHES RELATING TO 



though the points of emergence do not usually correspond ; they form 

 complete rings, in at least sonic Echiurians. The nerve-trunk usually 

 iloats in the body-cavity, moored by mesentery and trabecule; in 



some Thalassemias, however, it rests on a longitudinal muscular 

 elevation and has then lost its usual muscular sheath. The collar is 

 enormously elongated below the oesophagus; its two branches are 

 furnished throughout with ganglion-cells. The nerves end peri- 

 pherally in a subepithelial plexus, in relation with numerous 

 epidermal cells, generally localized in the papillae and not much 

 m< dified. 



7. The vascular system and body-cavity fluid. — The vascular 

 system consists (1) of a ventral vessel, disappearing posteriorly, and 

 forking anteriorly to form a ring round the proboscis, and (2) of a 

 dorsal vessel, beginning at the anterior extremity of the intermediate 

 intestine. The latter vessel has at first thick swollen muscular 

 walls ; it passes straight to the cephalic tube, opening into the above- 

 mentioned ring. Another variable anastomosis between the two 

 vessels occurs. In Echiurus the posterior extremity of the ventral 

 vessel opens into a vascular ring round the intestine, but the two 

 branches of this ring only unite at some distance beyond the in- 

 testinal walls. From this ring a vessel proceeds to form a second 

 ring round the interbasal muscle of the bristles, and then joins the 

 ventral vessel. This muscle-ring may, however, remain unclosed, 

 when its two branches open directly into the ventral vessel. One 

 might then say that the ventral vessel forms a ring round the inter- 

 basal muscle, and that the neuro-intcstinal anastomosis forms the 

 superior branch of this ring. A similar arrangement occurs in 

 B. minor, and in some Thalassemias, but the anastomosis is moved 

 backwards, and passes not into the ramification of the ventral vessel, 

 but into the vessel itself. At the same time, the intestinal ring round 

 the intestine becomes modified into a sinus. The free portion remains 

 normal, so that one sees two vessels issuing from the peri-intes- 

 tinal pouch. In B. viridis. they only unite just before passing into 

 the ventral vessel, while the two branches of the muscular ring, 

 which forms the ventral trunk, both pass below the interbasal muscle. 

 Hamingia resembles B. viridis. 



The vessels contain the amoeboid corpuscles which occur in the 

 perivisceral fluid, and also spherical elements, containing haemoglobin 

 in Thalassnna and Hamingia, and probably also in the Bonellias and 

 in Echiurus. 



8. The cephalic lube or proboscis. — The highly extensile and con- 

 tractile proboscis which is so characteristic of Echiurians is next 

 described. The elasticity of all its elements and of the amorphous 

 matrix in which they are imbedded, the abundance of muscles 

 crossing in all directions, the spiral coiling of the nerve-cords and 

 vessels, the very peculiar structure of the latter are all associated 

 with the marked changes of dimension which the proboscis exhibits. 

 The phenomena of extension are not due to an inrush of perivisceral 

 fluid. The lacunae which are seen in preparations beside the lateral 

 vessck of Thabisscma and Echiurus are evidently identical, they can- 



