190i.] NATURAL SCIENCES OF PHILADELPHIA. 747 



a few nuclei, of which four central ones are notably conspicuous. 

 It is traversed only by the duct (Gl.d.) of the gland (GL). It is 

 owing to the presence of this diaphragm that movements of the 

 proboscis cause no movements whatever of the fluid within the cavity 

 (archicoel) of the head-trunk. 



The proboscis is seen in evaginated condition in fig. 36, in contracted 

 in fig. 37. The evagination is due to the contraction of longitudinal 

 muscles (Mus.) lying beneath the body wall and reflected over the : 

 axial stalk of the proboscis. A contraction of these muscles evaginates 

 the armature, partly perhaps by a direct pull, partly perhaps by pro- 

 ducing a pressure upon the fluid contained ^^•ithin the cavity of the 

 proboscis. A comparison of figs. 36 and 37 furnishes the best repre- 

 sentation of the mechanics of this process. Wlien the armature is 

 evaginated the wall of the proboscis shows its annular folds ^-ery close 

 together (fig. 36); when retracted, further apart and less numerous. 

 \A^iat causes the retraction of the armature is difficult to decide, but 

 it is perhaps due to the tension of certain long cells (Fib.) lying at the 

 base of the axis. 



The hypodermis of the outer wall of the proboscis is exceedingly 

 thin from the diaphragm to the base of the third row of spicules (Sp. 3) 

 of the armature, and contaias not more than one or two nuclei. On its 

 outer surface is a thicker homogeneous cuticula which does not extend 

 over the armature region. Cuticula and hypodermis together compose 

 a very thin outer wall which is thrown into folds, so that the larva 

 appears to be superficially annulated. As well as I could determine 

 these folds are temporary, and their number changes with movements 

 of the proboscis, though the annulations are always present. There 

 are no circular muscles to produce them. The hypodermis is thickened 

 below the spicules and stilets which compose the armature, and shows 

 one nucleus at the base of each stilet and each spicule; at the base of 

 the stilets {St., figs. 36, 37) the hypodermis becomes continuous with 

 the axial ectodermic cell mass. 



The armature, best studied in life or on individuals treated with 

 caustic potash, shows three rows of spicules and three stilets (figs. 

 33-37). The first row of spicules {Sp. 1) is composed of a circle of 

 six spicules, namely, a pair of dorso-lateral and a pair of dorso-ventral 

 spicules, and a pair of lateral ones considerably larger than the others. 

 The second row {Sp. 2) is made up of a circle of four spicules, a dorso- 

 lateral and a ventro-lateral pair lying beneath the corresponding pairs 

 of the first row; these are the smallest of all the spicules. The spicules 

 of both of these rows are flattened, pointed cuticular plates, with 



