154 Introduction to A?ztmal Morphology. 



CHAPTER XXIII. 



SUB-KINGDOM 5. — ^VERMES. 



Bilaterally symmetrical, usually soft, elongate,, 

 often parasitic animals, of one or many metameres ; 

 with differentiated dorsal and ventral surfaces, but 

 never with jointed limbs. The nervous system gene- 

 rally consists of one or two pharyngeal ganglia, and 

 sometimes an abdominal nerve-cord. Sense-organs 

 are rarely complex. Sexual reproduction occurs in 

 every species ; metagenesis is common, but fission or 

 gemmation is rare. The ova are holoblastic, and the 

 embryo has rarely a primitive streak (except in 

 leeches). The epidermis may be cellular or proto- 

 plasmic, ciliated at first, but the cilia are in general- 

 early shed, and a deeper layer exposed, with bristles, 

 hooks, scales, &c. The dermis is fibrous or connective. 

 The subjacent muscular layer is either united to the 

 body parenchyma, or separated from the viscera by a 

 body cavity. There is a tubular water- vascular system 

 opening externally, and often also into the body 

 cavity ; this may be excretory, as Lieberkilhn found 

 guanin in its contents, but it is never connected with 

 locomotion. The mouth is at the end foremost in 

 progression, which may be specialized as a head. 

 The anus is farther back, or none. The higher forms 

 have a closed vascular system, but no pulsating sacs. 

 This polymorphic sub-kingdom may be divided 

 into two provinces, including thirteen classes.* 



* Some unite Turbellaria, Cotylidse, Nematelmia, and Acanthocephala 

 under the name Scolecida. Gephyrea has been joined to Echinodermata, 

 to Scolecida, or Annehda. Hirudinea has been placed beside or under 



