♦ 



igog.] F. H. Stewart : Investigator sicarms. 285 



differs in no way from that found in other cœlomate worms. In situations where it 

 is reflected on itself at a fairly acute angle, as at the junction of the coronal partition 

 and the body-wall, the endothelial cells are somewhat heaped up and less flattened, 

 and they closely resemble the free cells of the body-cavity, in fact it seems clear that 

 the cells of the latter are derived from the endothelium. These free cells have a 

 clear protoplasm which does not stain, a sharp round outline and an intensely stain- 

 ing nucleus. They are identical with those found in the dorsal blood-vessel. 



The body-cavity is divided throughout the entire length of the neck and trunk 

 by a fibrous and endothelial partition, lying in the coronal plane of the animal (pi. 

 xxi, figs. 5, 6, 7). This partition does not extend into the head. Of the two divisions 

 into which it divides the body-cavity, the dorsal is by far the largest and contains all 

 the viscera, — alimentary canal, liver, nephridia, testis, and the main dorsal blood- 

 vessel. In the region of the neck some of the sacculi of the foregut pass through 

 the partition and project into the ventral division; the ventro-lateral nerve cords also 

 project into it in this region, but with these two exceptions it is empty. 



In the hind region of the neck and the front of the trunk, before the liver 

 appears to occupy the greater portion of the body-cavity, there are in addition to the 

 coronal diaphragm two pairs of dorso-lateral longitudinal partitions running from the 

 body- wall to the main dorsal blood-vessel and the gut (pi. xxi, fig. 6). It has not 

 been possible to trace any communication between the body-cavity and either the 

 exterior or the cavities of the testis or nephridia. 



In regard to the nature of the body-cavity, the question arises is it cœlom or 

 not ? Without the aid of embryology it may not be possible to give a very definite 

 answer to this question. The main argument against its cœlomic nature would be 

 the fact that the cavity of the gonad does not communicate with it, but it seems to 

 be fairly generally admitted that the body-cavity of Priapulids is a cœlom from 

 which a special part has been shut off to form part of the lumen of the reproductive 

 tube. Now the Priapuloidea are the group to which Investigator is most closely 

 allied, and the same interpretation might therefore be very reasonably applied to 

 Investigator. But again the presence of the same type of free cells in both blood- 

 vessels and body-cavity might suggest that the cavity was haemocœlic, but lymph 

 cells are known to pass freely into the blood stream (as, for instance, by the thoracic 

 duct of Mammalia), so that no importance can be attributed to the distribution of 

 these cells. 



The fact that the nephridia project freely into the cavity is in favour of regard- 

 ing it as cœlom. Altogether the balance of evidence seems to be in favour of this 

 view, and there is certainly as much reason to suppose that it is cœlomic in Investi- 

 gator as in the Priapuloidea. 



The A1.1MENTARY System. 



The ahmen tary canal, extending from the slit-like mouth at the anterior extrem- 

 ity to the anus opening into the gill-chamber underlying the shield, can be divided 

 into pharynx, foregut and hindgut. 



