Characteristics of Nematelminthes. cxxxv 



tional rather than depuratory, as the water-vascular system of the 

 Infusoria is supposed to be by some authors. 



The nervous system in the Chaetognafha consists of a sing-le 

 ventral and a single prae-oral gang-lion, connected by commissures, 

 so as to form a collar round the oesophag-us ; in the Nematoidea 

 we have a fibrous ring", wdth ganglion cells interspersed in its sub- 

 stance, surrounding the oesophagus^ and connected posteriorly with 

 three main aggregations of ganglionic cells. Of these three ag- 

 gregations the first is placed medioventrally, and consists of two 

 symmetrical masses^ one of which is on either eide the middle line ; 

 the other two are placed on either side in the substance of the two 

 lateral bands; and the three ganglionic centres correspond thus 

 with the triradiate cej)halic lobes of some families, and the tri- 

 quetrous division of the oesophagus of most Nematoids. In the 

 Acanihocephali there is only a single ganglion, placed at the base 

 of their proboscis, and resembling thus the structure which has 

 been stated to exist in the anterior extremity, the so-called ' head ' 

 of the Taeniadae. In the free Nematoids, certain aggregations 

 of pigment granules, situated on the dorsal surface of the oeso- 

 phagus, are sj)oken of as ocelli, but no nerve filaments have been 

 traced into continuity with them, as there have been in the case of 

 the similar organs in the Chaeiognaiha. The cephalic lobes when 

 present, as also certain papillae developed on the ventral surface in 

 many Nematoids, are considered to be tactile organs. 



With the exceptions furnished by the order Chaetognatha , and 

 the Nematoid genus Pelodijtes (Schneider), all Nematelminthes are 

 dioecious. Their generative organs resemble those of the Platyel- 

 minthes and Discophora, in possessing an intromittent apparatus ; 

 and, with the exception of the female Acanthoo'fliali, whei-e the 

 ova escape by dehiscence into, and are taken up by an infundibuli- 

 form oviduct from the perivisceral cavity, the Nematelminthes 

 resemble the Rotifera, Discophora, and Platyelminthes, and differ 

 from the Annulata in having the walls of their generative ducts 

 continuous with the envelopes of their generative glands. The 

 ovaria are greatly developed, and the ova very small and numerous 

 in the parasitic Nematoids, the reverse being the case with the 

 free species. The spermatozoa of the parasitic Nematoids are 

 spherical or ovoidal cells, and move only by the protrusion of 

 pscudopodial protoplasmic processes; in the free genera they are 



