Characteristics of Vermes. cxxiii 



distinctly divided externally by annulation;, and internally by the 

 development of dissepiments forming compartments. The nerve- 

 ganglia, the organs of motion, and, sometimes, even those of special 

 sense, exhibit a more or less closely corresponding multiplicity as do 

 also the organs of vegetative life. The Annulata are divisible into 

 two Classes, the Annulata proper s. Annelides and the Gephyrea. 

 The Annuloida are unisegmental Vermes or 'aggregates of the 

 second order.^ Their nervous system consists (with, possibly, an 

 exception in the case of certain Turbellarians, see p. 155, infra) at 

 most of a simple oesophageal collar to which a few accessory nerve- 

 centres, but now a chain of ganglia may be appended; and their 

 water-vascular or depuratory system is (with an exception again in 

 the case of certain Turhellaria, the Nemertinea, see description of the 

 class Platyelminthes given below, p. cxli.) the only vascular system 

 which they possess. The Division Annuloida contains three Classes, 

 the Nematelminthes, the Rotifera, and the Platyelminthes. 



The integument may possess a perfectly smooth chitinous ex- 

 terior; or it may be covered with cilia; or it may develope nu- 

 merous chitinous outgrowths in the shape of spines, hooks, bristles 

 or hairs. These appendages often attain a very considerable degree 

 of hardness ; but the chitinized cuticular secretions of the body-wall 

 as opposed to these specialized developments which often pass 

 right through the thickness of the visceral envelope, are usually 

 much less resistent both to physical and to chemical agencies than 

 the similarly placed structures of the Arthropoda; they never 

 become indurated by calcificatory deposit, and have only rarely 

 (in Nematoidea and Hirudineae) been observed to be changed by 

 moulting as in that sub-kingdom. In some of the ento-parasitic 

 Vermes [Cestodes), which lie immersed in an atmosphere of more 

 or less perfectly digested and diffusible albumen, and which by 

 virtue of their readily permeable integument can imbibe nutriment 

 at all points of their external surface, there is absolutely no digestive 

 system present. In the parasitic Nematoids, on the other hand, in 

 which the integument developes a much more considerable chitinous 

 cuticular deposit, a proctuchous digestive tract is always present. 

 The non-parasitic Vermes, with the exception of the male E-otifera, 

 always possess a digestive tract, which is usually proctuchous, except 

 in the case of the Dendrocoelous Turhellaria, which in this as in 

 other points resemble the parasitic order Trematodes. 



