TYPE NEMATHELMIXTHES. 175 



female directly to the exterior, in the males into a cloaca 

 common to it and to the male organ of reproduction. Its 

 anterior part is a muscular oesophagus lined with cuticle di- 

 rectly continuous with that covering the surface of the body, 

 while posteriorly it is a delicate tube composed of a single 

 layer of cells, not being surrounded by any rnesodermal mus- 

 cular tissue. 



The excretory system is not as yet fully understood. It 

 appears to consist of a pair of tubes, for which no cellular 

 lining has as yet been made out, which lie, one on each side, 

 in the thickened hypodermis of the lateral lines. In the an- 

 terior portion of the body they unite to form a single tube 

 which opens to the exterior in the median ventral line not far 

 behind the brain (Fig. 88, B). 



This latter consists of a ring or nerve-collar surrounding 

 the anterior part of the oesophagus on which lateral masses of 

 ganglion-cells occur and which gives rise to two main nerves, 

 one of which runs back in the median dorsal line, while the 

 other, which in some forms appears to be double, lies in the 

 median ventral line. Other nerves pass forwards from the 

 nerve-ring to the anterior part of the body, and in addition to 

 the dorsal and ventral nerve-cords two lateral nerves pass 

 backwards a short distance, while circular commissures con- 

 nect the two main nerve-cords, those of the two sides of the 

 body not, however, being opposite each other, so that they do 

 not suggest a pseudo-metamerism so strongly as the similar 

 commissures of the Hoplouemertiui. Special sense-organs 

 are as a rule absent, though a few forms possess eyes. 



The reproductive organs are exceedingly simple. In the 

 male they are represented by a single convoluted tube, lined 

 in its upper part by the mother-cells of the spermatozoa and 

 dilating below into a seminal vesicle, to which succeeds a 

 short ejaculatory duct which opens into the cloaca. The walls 

 of this latter cavity are frequently invagiuated to form two 

 small sacs in each of which lies a chitiuous spicule capa- 

 ble of being protruded from the cloacal opening and serving, 

 with the bursa, as copulatory organs. The female organs, on 

 the other hand, consist of a pair of convoluted tubes, each of 

 which dilates into a uterus and unites with its fellow to form a 



