194 



COMPARATIVE ANATOMY 



CHAP. 



{ArcMannelida) wanting. Other layers are, however, sometimes added 

 to the two typical layers, for instance, a layer of fibres which cross 



each other diagonally in the 

 Hirudinea and Echiuridce. The 

 circular layer is everywhere 

 continuous ; the longitudinal 

 layer, however, is almost al- 

 ways broken through at dif- 

 ferent places in the ChcBtojjoda. 

 These breaches are often very 

 dissimilar in different genera 

 and families, so that they 

 cannot all be comprised in the 

 same description. The most 

 frequent are those in the dor- 

 sal and ventral lines, then in 

 the longitudinal lines formed 

 by the bundles of setae and the 

 parapodia. The various ar- 

 of these latter 

 naturally causes variety in the 

 arrangement of the lines or 



Fig. 127. — Transverse section through a Nema- 

 tode (Ascaris). dn, Medio-dorsal, vn, medio-ventral rangement 

 longitudinal nerve in the line which represents the 

 middle line of the body ; si, lateral lines ; c, cuticle ; 

 ^y, hypodermis ; S(/, lateral vessels ; ov, ovarial tubes ; 



/m, longitudinal musculature ; Imk, cell elements of qvoq g Qf interrUDtion As a 

 the longitudinal muscular fibres ; -a, uterus ; md, i ■ , i v 7 , , i i 



^roi^.gut. rule m the Annmata, the longi- 



tudinal musculature is more 

 strongly developed than the circular musculature. 



The dermal musculature of the Myzostomidce is difficult to make 

 out. We may perhaps distinguish : (1) a system of fibres which 

 radiate from the centre to the circumference; (2) a system of fibres 

 concentrically arranged, and running parallel to the edge of the body: 

 The first system must represent the circular musculature of the Annii- 

 lata, the second their longitudinal musculature. 



The various groups of muscles which serve in the Chcetopoda for 

 moving the bundles of setse, the parapodia and their appendages, must 

 be regarded as special local modifications of the dermal musculature. 

 In the SternaspidcB, as in the Sipunculidw, we find parts of the longi- 

 tudinal musculature difierentiated into dorsal and ventral retractors of 

 the anterior introvertible portion of the body. 



The general body musculature is developed in a very different 

 manner in the Prosopygia. The naked Sipunculacea possess a strong 

 and typically developed dermo-muscular tube, consisting of an outer 

 circular and an inner longitudinal layer (Fig. 128). Between 

 these two, in the Sipunculidce,, a thin layer of diagonal fibres is inter- 

 posed. The longitudinal and circular muscles generally run in 

 regular bundles or bands lying side by side, and these correspond 

 with the outwardly visible longitudinal and circular ridges. The 



