VERMES. 579 



somites is distinct in Chaetopoda and Archi- Annelida ; present but obscured 

 by a secondary annulation of the somites in Hirudinea ; probably aborted 

 in Gephyrea. It is indicated by the presence at regular intervals of bundles 

 of dorso-ventral muscle-fibres, &c., in Nemertea and Triclad Turbellaria. 

 The Enteropneusta retain a division of the body into two regions, a collar 

 and trunk, in correspondence with the mode of development of the 

 coelome. The segmentation of the strobila in Cestoda is seemingly corre- 

 lated solely with reproductive necessities. Other Vermes are certainly uni- 

 segmental. A large prae-oral lobe or prostomium is present in Entero- 

 pneusta and Gephyrea chaetifera. The corresponding region in Chaetopoda, 

 Archi- Annelida, and Hirudinea is much reduced, and may in some cases be 

 an outgrowth of the peristomial segment. 



The coelqme may be large, and is sometimes divided into compart- 

 ments in segmented Vermes ; it may be represented by irregular passages 

 between the mesoderm cells, or even absent (?) altogether as in Acoelous 

 Turbellaria. In the Hirudinea it undergoes obliteration (diacoelosis) to a 

 very great extent during growth, and may be partially replaced by spaces 

 secondarily formed (metacoelosis) in the mesoderm. It opens externally 

 by special pores in many oligochaete Chaetopoda. In \he.Enteropne^^sta and 

 ChaetognatJia it is an enterocoele developed from the archenteron ; in the 

 segmented Vermes, and probably in Nematoda, it is a schizocoele ; and in 

 Rotifera it appears to be a permanent archicoele. The coelomic channels 

 of Trematoda, &c., are apparently intercellular spaces \ 



The characters of the integument and disposition of the muscles of the 

 body-wall are subject to much variation. It may be noted that the ecto- 

 derm, hypodermis or epidermis cells are transformed into the cuticle of 

 Trematoda. It is uncertain whether or not the same is the case in Cestoda 

 and Acanthocephala, but in the first-named the hypodermis is perhaps 

 represented by sub-cuticular cells. 



The nervous system may retain a position in the hypodermis ( = ectoderm) 

 as in Enteropneusta, some Chaetopoda, Archi-Annelida, Gephyrean Priapu- 

 lidae, some Nemertea, Chaetognatha, and to a certain extent in Nematoda. 

 In the Turbellaria the central ganglia do not always bear any direct rela- 

 tion to the mouth or pharynx ; and in Nemertea the commissures of the 

 cephalic ganglia surround the proboscis and not the oesophagus. A peri- 

 pharyngeal union of the cephalic ganglia is exceptionable in Trematoda. 

 The ventral nerve-cords are segmented more or less distinctly in those 

 Vermes in which the segmentation of the body is pronounced. Numerous 

 longitudinal nerve-trunks of almost equal importance are found in Trema- 



1 Fraipont's view as to tfia existence of intercellular coelomic spaces or channels in Cestoda 

 and Trematoda is opposed by Pintner, who holds that, in some Cestoda at least, there is a system of 

 intracettular canals with which the basal processes of the flame-cells are connected. The former 

 view seems more likely to prove correct. 



P p 3 



