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 coelome 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 the Enteropneusta and 

 Chaetognatha 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 Priapti- 

 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 the 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 

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

 view seems more likely to prove correct. 



P p 2 



