THE ORIGIN OF ELYTRA. 63 



smaller than the ventral or neuropodium, and its setae are shorter 

 and of a different pattern to those of the latter. The drawings 

 on PL XI sufficiently illustrate the general arrangement and the 

 details of divergence. Besides the ordinary setae, each branch of the 

 parapodium contains a single stout spine, aciculum, buried, all but 

 the point, in the flesh. 



The form of the parapodia often termed rather loosely- the 

 feet and of their bristles, is remarkably stable throughout the whole 

 length of the body, excepting only in the case of the first somite, the 

 peristomium, the one bearing the mouth. Here each parapodium 

 is reduced to a long slender lobe, obscurely divided into rudimentary 

 upper and lower branches, and with but one aciculum and two or 

 three tiny seta3. 



The tactile appendages garnishing the various somites show 

 much divergence. Thus the somites 2, 4, 5, 7, 9, 11, 13, 15, 17, 19, 

 21, 23, 26, 29, 32, bear each one pair of the characteristic scales, or 

 elytra, while the remaining ones bear on either side a long filiform 

 appendage, the dorsal or notopodial cirrus, arising from the base of 

 the notopodium. Reference to Fig. 1 will explain this arrangement 

 and show how, in the major part of the body, these two organs 

 roughly alternate, giving place in the last ten somites to cirri only, 

 thus leaving this short portion naked. The neuropodium bears also a 

 short ventral cirrus, but this shows no variation except in the 

 peristomium, where it is greatly elongated. 



In structure, the elytra are thin membranous two-layered plates, 

 borne on broad low peduncles, and so loosely attached, that it is 

 extremely difficult to obtain a preserved specimen with the full 

 number present. This extreme looseness is peculiar to this species. 

 In others, such as the common Lepidonotus squamatus, they adhere 

 with such firmness that they frequently tear rather than lift off, and 

 between these two extremes, there is every intermediate degree. 

 The surface of the scale is prettily marbled, while around the exposed 

 edge is a row of all but globular tubercles, very diagnostic of this 

 species. Occasionally I have noticed the elytra to be distended in a 

 bladder-like manner. Some observers have supposed this to be an 

 incubatory device, as they have found them filled with ova. This 

 however does not necessarily follow, for the ova when freed, find their 

 way into all the spaces of the body-cavity, and of this, the hollows of 

 the elytra are but offshoots. 



Probably elytra are modifications of certain dorsal cirri the two 

 never being found co-existent upon the same somite in Polynoe, 

 though in Sigalion, a scale-covered worm of great length, they do 

 exist together. In the latter case however, the common phenomenon 

 of duplicative variation (see Bateson's " Materials for the Study of 



