I .' 



92 ALICE L. EMBLETON ON THE STRUCTURE AND 



Danielssen and Korcn, in Epithetosoma norvegicum, speak of a ventral mesentery ; 

 thongh Sedgwick *, referring to tlie Echiuroidea as a wliole, says " there is no special 

 mesentery, bat strands of tissue run from all parts of the body-wall across the body-cavity, 

 to be inserted into the walls of the alimentary canal." 



Jameson, however, in his recent paper on Thalassema ueptimi, recognizes a dorsal 

 mesentery running along the whole course of the alimentary canal except on the gizzard, 

 and a ventral mesentery, " bearing the ventral vessel througliout its entire length, is 

 attached to the alimentary canal anteriorly and posteriorly." ... "At the hinder end of 

 the oesopliagus it suddenly leaves the digestive tube with a falciform free margin and 

 extends as a short free fold along the ventral surface, attached to the peritoneal sheath 

 of the nerve-cord and bearing the ventral vessel. Posteriorly it approaches the gut 

 again, and attaches itself to the csecum, on the walls of which the solid continuation of 

 the ventral vessel ends." ..." Owing to the presence of the web of ventral mesentery 

 which occupies the oesophageal loop, the gut, with the two mesenteries, shuts off a 

 2)ortion of the body-cavity forwards, forming a kind of false diaphragm. This anterior 

 division of the body-cavity opens out by a single small hole through which the ventral, 

 and at times the dorsal, blood-vessel passes forwards ; this aperture is bounded by a 

 small piece of the left ventral body-wall wliich lies between the insertion of the 

 dorsal mesentery and the nerve-cord." ..." Tiie radially arranged folds of peritoneum 

 around the mouth and anus are quite irregular and consequently call for no attention." 

 I dissected Thalassevia nephmi and Echinrus Pallasii in order to see the mesenteries, 

 and conclude that wjiat Jameson calls "a kind of false diaphragm" corresponds to what 

 Spengel calls the "diaphrgam" in Ecitmrus Pallasil. But in Echiurus unicinctus there 

 is no structure comparable with either of these membranes ; the only mesenteries are 

 irregular strands attaching the alimentary canal and anal vesicles to the body-wall, and 

 the radiating mesenteries on the rectum and buccal sac. PI. 10. fig. 39, is a drawing of 

 the worm as it appears when first opened with the alimentary canal not yet uncoiled ; 

 obviously the arrangement of the mesenteric strands is quite irregular and indefinite {s.ni.). 

 The arrangement of the peritoneal folds on the buccal sac is shown in fig. 1, p. 91, Avhere 

 many of the dorsal attachments are shown cut through. Figs. 4.0-43 are drawings of a 

 series of sections cut through the anterior end of the body, in wdiich the radial folds 

 appear in transverse section ; they are most numerous dorsally, fixing the pharyngeal 

 portion of the alimentary canal fivmly to the body-wall. 



As regards mesenteries, therefore, it appears that Echiurus unicinctus differs widely 

 from Echiurus Pollasii and Thalassema neptuni. There is no indication of segmentation 

 that I can see, in the structure of the adult of this Japanese Echiurus ; as to its probable 

 relationship Avith the segmented worms, that is a question which can only be settled after 

 a careful study has been made of the development of this and allied species, and by 

 following the fate of the larval organs. It is known in the trochophore larva of Echiurus 

 that the pair of mesoblastic bands become segmented up into 15 pairs of mesoblastic 

 somites, though, so far as is known at present, no segmentation occurs in the larva of 

 Eonellia. 



'■ * student's Text Book of Zoology, vol. i. 1898, p. 529. 



