Characteristics of Vermes. cxxv 



cavity; and in certain segments these organs are modified, so as 

 to serve as efferent ducts for the generative products. 



The existence of a nerve-system appears to be doubtful in cer- 

 tain of the Platyehninthes (Cestodes). In the parasitic Accmtko- 

 cephali it is reduced to a single anteriorly-placed ganglion. In 

 certain both of the free and of the parasitic Platyelminthes^ it 

 consists of two ganglia placed one on either side of the pharynx 

 {Trematodes), or one on either side of the anterior extremity of the 

 body {TurhelJaria), and connected with each other by a transverse 

 commissure. The ordinary Nematoidea possess a complete circum- 

 oesophageal collar^ which again is represented in the microscopic 

 Rotifera by a single bilobed supra-oesophageal ganglion. The 

 Gephyrea have in addition to the nerve-collar of lower Vermes a 

 simple elongated band of nervous tissue, extending from the an- 

 terior to the posterior pole of the body along its medio- ventral line. 

 In the Annulata proper the ventral chain is distinctly bilaterally 

 symmetrical, and its two halves are sometimes widely divaricated. 

 In many Vermes the organs of special sense (optic and auditory) are 

 very well developed ; in many, they are entirely absent. 



In the Annulata proper, with the exception of the DlscopJiora, the 

 ova and spermatozoa are set from the secretory glands, by dehiscence 

 into the abdominal cavity^ and are conveyed thence to the exterior 

 by modified ' segmental organs.' In the other Vermes, with the 

 exception of the female Acanthoce])hali, the generative glands 

 have their walls prolonged into ducts ; and their products are thus 

 conveyed out of the body without falling into the perivisceral 

 cavity. Structures corresponding to the intromittent organs of 

 higher animals are found in representatives of every class of Vermes, 

 not even exclusively of the Rotifera; the Platyelminthes almost 

 invariably possess a complicated reproductive apparatus, in which, 

 besides other accessory organs, vitelligenoiis exist independently of 

 germigenous glands. The marine Annulata, on the other hand, 

 are distinguished by a great simplicity in their reproductive appa- 

 ratus, contrasting herein at once with the lowest Vermes, and with 

 the Discophorous and Oligochaetous members of their own class. 



Vermes may be either hermaphrodite or dioecious ; either vivi- 

 parous or oviparous ; they may reproduce their kind either sexually 

 or asexually, and their embryos may or may not go through a 

 metamorphosis. When reproduction takes place asexually, it may 



