Each fully developed organ consists of three parts:
(1) A dilated vesicle opening externally at the base of a foot.
(2) A coiled glandular tube connected with this and subdivided again into several minor divisions.
(3) A short terminal portion opening at one extremity into the coiled tube (2) and at the other, as I believe, into the body-cavity. This section becomes very conspicuous in stained preparations by the intensity with which the nuclei of its walls absorb the colouring matter.
The segmental organs of Peripatus, though formed on a type of their own, more nearly resemble those of the Leech than of any other form with which I am acquainted. The annelidan affinities shewn by their presence are of some interest. Around the segmental organs in the feet are peculiar cells richly supplied with tracheæ, which appear to me to be similar to the fat bodies in insects. There are two glandular bodies in the feet in addition to the segmental organs.
The more obvious features of the nervous system have been fully made out by previous observers, who have shewn that it consists of large paired supra-œsophageal ganglia connected with two widely separated ventral cords—stated by them not to be ganglionated. Grube describes the two cords as falling into one another behind the anus—a feature the presence of which is erroneously denied by Saenger. The lateral cords are united by numerous (5 or 6 for each segment) transverse cords.
The nervous system would appear at first sight to be very lowly organised, but the new points I believe myself to have made out, as well as certain previously known features in it appear to me to shew that this is not the case.
The following is a summary of the fresh points I have observed in the nervous system:
(1) Immediately underneath the œsophagus the œsophageal commissures dilate and form a pair of ganglia equivalent to the annelidan and arthropodan sub-œsophageal ganglia. These ganglia are closely approximated and united by 5 or 6 commissures. They give off large nerves to the oral papillæ.
(2) The ventral nerve cords are covered on their ventral side by a thick ganglionic layer[462], and at each pair of feet they dilate into a small but distinct ganglionic swelling. From each ganglionic swelling are given off a pair of large nerves[463] to the feet; and the ganglionic swellings of the two cords are connected together by a pair of commissures containing ganglion cells[464]. The other commissures connecting the two cords together do not contain ganglion cells.