194 



COMPARATIVE ANATOMY 



CHAP. 



(Archiannelida) 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 Chcetopoda. 

 These breaches are often very 

 sl 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 



FIG. 127. Transverse section through a Nema- parapodia. The Various ar- 



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

 longitudinal nerve in the line which represents the -1-1 , .-, 



middle line of the body; sl, lateral lines; c, cuticle; naturally caUSCS Variety in the 



hy, hypodermis ; sg, lateral vessels ; ov, ovarial tubes ; arrangement of the lines Or 



Im, longitudinal musculature ; Imk, cell elements of areag Q f interruption. As a 

 the longitudinal muscular fibres ; u, uterus ; md, , . . , A 7 r . , , , 



mid-gut. m l e ln tne 4nnulata, the longi- 



tudinal musculature is more 

 strongly developed than the circular musculature. 



The dermal musculature of the Mijzostomidce 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 Annu- 

 lata, the second their longitudinal musculature. 



The various groups of muscles which serve in the Chcetopoda for 

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

 be regarded as special local modifications of the dermal musculature. 

 In the Sternaspidce, as in the Sipunculidce, we find parts of the longi- 

 tudinal musculature differentiated 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 longitudinaj and circular ridges. The 



