OF THE NEMATOIDS, PAEASITIC AND FREE. 023 



we have strong enclence for the belief that the excretoiy gland, opening extei-nally in 

 the oesophageal region of many Nematoids in which, so far as yet observed, the lateral 

 vessels arc wanting, is tlie rudimentary homologuc of this system of canals, and may be 

 considered as a permanent record of a transitory stage in their development in other 

 species, as well as in that of the more complex form of this apparatus usually met with 

 amongst the Echinoderms and Scolecids. An objection that may perhaps be urged to 

 the homology of this apparatus in the Nematoids with that of the ambulacral system in 

 the Echinoderms, is, that the latter system opens in the dorsal region of the body, whilst 

 the former has its exit in the ventral. To this I would reply, that, as in the Echinoderms, 

 this system opens in the Nematoid on the same aspect of the body as the genital organs 

 and the rectum, and considering the bilateral symmetry of these latter animals together 

 with the terminal position of their mouth, it is a matter of indifference for the purposes 

 of transcendental anatomy which surface of the body is considered as dorsal and whicli 

 as ventral : but seeing that these systems in the Echinoderms open undoubtedly 

 upon the dorsal aspect of the body, and that the cloacal aperture of the same system is 

 in a similar situation in the llotifera (one of the groups of Scolecida), it seems pretty 

 evident that in a developmental and homological point of view, that which has hitherto 

 been spoken of as the ventral aspect of the Nematoids should be considered in reality as 

 the dorsal. So far as the structure of the nei-vous system of the Nematoids is concerned, 

 it also lends support to this view, since undoubtedly a much larger number of ganglion- 

 cells are situated on what has hitherto been described as the ventral part of the oesophageal 

 ring than can be met vdth. in connexion with its opposite half. But the preponderance 

 of ganglionic nervous matter in an oesophageal ring is usually 07i instead of beneath the 

 oesophagus. The position of the ocelli in the free Nematoids is the only fact seeming 

 to militate against this view; but I think this objection cannot be allowed mucli weight, 

 seeing that the ocelli appear to be comparatively trivial organs which may be present or 

 absent even in species of the same genus. 



In the Echinoderms and in certain genera of free Nematoids there remain, however, 

 certain other vessels which have not yet been accounted for. In the former animals I 

 allude to that apparatus of vessels which is generally described as their "vascular 

 system," and concerning which our knowledge is still somewhat defective. It is not 

 absolutely proved that this is quite distinct from the ambulacral system, and our 

 knowledge of the way in which it is developed is also obscure. Nothing more definite 

 can at present be said of the simple lateral vessels existing amongst the free Nematoids 

 in the genera Tylenclms, Aphelenchus, and Plectus, I could only ascertain that such 

 vessels exist, and in Tylenclms at least, without the shadow of a doubt, that they float 

 free from the integuments along either side of the abdominal cavity. Whether they 

 communicate with one another, or with the exterior could not be ascertained, though, as 

 before stated, there is slight evidence to show that in the genus Plectus these tubes do 

 communicate with the exterior, each by means of a lateral, cervical, in tegumental pore*. 



• See note, page 619. 



