240 EDWIN S. GOODEIOH. 



The thick brown ventral shield is insoluble in boiling water, 

 caustic potash, alcohol, and ether. Placed in cold concen- 

 trated hydrochloric acid, it gives an orange-yellow solution ; a 

 transparent colourless portion remains, which dissolves with 

 difficulty on boiling. 



The chsetse are insoluble in water, caustic potash, alcohol, 

 and ether. As in the case of the earthworm (2), an outer 

 shell and distal cap remain undissolved when they are boiled 

 in hydrochloric acid. 



The shield, then, is probably formed of the same substance 

 as the chsetse — not of true chitin. 



The Muscular System. — Since the musculature of Ster- 

 naspis has been only very slightly dealt with by previous 

 observers, a somewhat detailed account of the highly modified 

 muscular system of this Polychsete is here given. It is well 

 known that, when irritated, Sternaspis can rapidly withdraw 

 the first seven segments into its hind body ; this introversion 

 is brought about by a complicated system of muscles, derived 

 chiefly from the longitudinal layer. 



A side view of an expanded specimen is showu in fig. 16, 

 and of a retracted specimen in fig. 15. In the latter the 

 whole of the anterior region has been withdrawn to the level 

 of the genital papillse {gen. p.), which remain projecting from 

 the front edge of the body. The branchiae at the hind end are 

 not withdrawn, but merely contract into close spiral coils. 

 Their contraction is, however, quite independent of that of the 

 body. 



Externally we notice the first segment, bearing the mouth 

 {m.) and the rounded prostomium (prost.), followed by seg- 

 ments 2, 3, and 4, armed with strong chsetse. Each row of 

 chsetae is set on a crescentic and slightly elevated area on each 

 side at the hinder margin of the segment. As has been shown 

 by previous observers, the bundles of chsetse in segments 8 to 14 

 are sunk in the body-wall, and completely hidden from view. 

 The grooves separating the first seven segments are shallow, 

 and completely surround the animal ; those separating the 

 posterior segments are deeper (in the expanded worm), and do 



