TYPE ANNELIDA. 205 



cations of the cirri or brandies arising from them, and 

 respiratory in function, also occur. Muscles pass from the 

 body-wall to the parapodia, which thus 

 become important organs of locomo- 

 tion and in some of the actively swim- 

 ming species assume a more or less 

 flattened plate-like form. 



The head segment is generally well 

 differentiated from those which succeed 

 it, being destitute of parapodia and 

 setsB, and as a rule carrying a number 

 of appendages sensory in function, and 

 being likewise usually provided with ^^ 



eyes. The cephalic appendages may ^^<*- 98^-PAnAPODroM of 



■^ ^ ^ ^ ° . Nereis mrens. 



be short and rather stout, forming 



° Jr = bran cilia. 



what are termed palp% (Fig. 100, p), ae = dorsal cinus. 

 or somewhat longer and more slender, s = seise. 



forming the drri (c), or even still more ""^ = ventral cirrus. 

 slender, being then known as tentacles (t). 



The body is enclosed in a chitiiious covering secreted by 

 the subjacent ectoderm, here known as the hypodermis (Fig. 

 99, hy). The musculature of the body-wall which lies below 

 the hypodermis is separated from this by a basement-mem- 

 brane and consists of an external layer of circular fibres {cm) 

 and a subjacent layer of longitudinal fibres ilm) which is, as 

 a rule, interrupted in the mid-dorsal and ventral lines and 

 also in the region of the two lobes of the parapodia so as to 

 form four bundles. Special muscles extend from the body- 

 wall to the base of the seta-sacs, and furthermore a pair of 

 muscle-bands cross the cavity of each metamere, in typical 

 cases passing from the lateral regions of the dorsal surface 

 downwards and inwards to be inserted into the ventral body- 

 wall on each side of the median line. The inner surface of 

 the longitudinal muscle-layers is lined with a layer of peri- 

 toneal cells which completely enclose the coelom (co) of each 

 metamere, being reflected upon the surfaces of the dissepi- 

 ments which form the internal partitions between adjacent 

 metameres. The separation of the ccelomic cavities of the 

 metameres is, however, rarely perfect, openings occurring here 



