MICROSCOPICAL STUDIES. 101 



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 

 Variation ") has probably been at work ; if so, then Polynoe shows 

 regular modification from a form possessing dorsal cirri to every somite, 

 while Sigalion shows such modification with duplication superadded. 



The head, the prostomium, consists of all to the front of the 

 mouth-somite, and indeed is apt to be confounded in part with the 

 latter, as its upper surface is bent back and lies partly obscuring it. 

 Viewed from above, after the concealing first pair of elytra are 

 removed, it appears sharply defined and roughly heart-shaped, with 

 two pairs of black eyes set widely apart (Fig. 3). Long appendages 

 of sensory function, borne on separate peduncles, are given off from 

 the fore edge. Four are in pairs, while a median unpaired one, the 

 median antenna, is given off, rostrum-like, from the central point. 

 On either side of this is a rather short, almost conical process, the 

 superior-lateral antenna ; while attached immediately beneath each 

 is a stout, very extensile organ, the inferior-lateral antenna or palp. 



In addition to these, the dorsal and ventral cirri of the peris- 

 tomium have to be considered as supplementary head appendages, as 

 they are specially elongated and arc directed forwards at the side of 

 the true appendages, acting in unison in sensory duties. They 

 receive the distinctive name of tentacular cirri. 



From the paired nature of the prostomial organs, it has been 

 suggested that, as happens among the Crustaceans and the Insects, 

 the head has resulted from the coalescence of several somites, which 

 in losing locomotive duties have been profoundly modified to subserve 

 sensory uses. But a very serious objection to this is, that all the 

 prostomium results from one particular part of the larva, known as 

 the prse-oral lobe, and which at no period of its history shows any 

 suggestion of segmentation. 



Turning now to internal arrangement, we find that the mouth 

 leads into a well-marked pharynx, the anterior portion of whose walls 

 lie in puckered folds capable of being everted like the finger of a 

 glove, and thus allowing of the protrusion of the hinder portion, so as 

 to form a proboscis. This hinder portion has walls of great thickness 

 and muscularity, and the anterior end when protruded, is crowned 

 with a circlet of conical transparent papillae, each containing a dark 



