THE ENTEEOPNEUSTA. 15 



tact with the ectoderm. These cells are in some parts only 

 one layer thick (anterior and posterior walls) (fig. 8), while in 

 others (ventrally) they proliferate rapidly (fig. 9), forming loose 

 masses of cells, the outer elements of which are elongated, 

 with rounded heads from which the spherical cells which form 

 the inner portion are budded. This proliferation continues 

 until the proboscis cavity is partially filled up. But in the 

 later phases of Stage F elements are formed other than the 

 rounded cells above mentioned, in the shape of fibres (fig. 13) 

 which appear to arise in a curious way, as is shown in figs. 9, 

 10, 12, &c. The elongated cells gradually become pyriform, 

 the round ends being for the most part central, while the fine 

 ends are drawn out into peripheral fibres. The round heads 

 then appear to separate from the fibres, so that in examining 

 the mesoblastic structure lining this cavity in late larvae of 

 Stage G, the elements are arranged in the following order : 

 centrally, a small empty cavity surrounded by a ring of sphe- 

 rical granular cells; next a layer of pear-shaped cells con- 

 tinued into peripheral fibres, and externally a layer, composed 

 almost, and later in life entirely, of radial fibres. Some of 

 these are inserted into the lower layer of the skin, and are 

 probably a peculiar form of connective tissue. In the heads of 

 the pyriform cells brightly refractive granules may be seen; 

 whether these are food or waste products cannot be affirmed. 



Such then is the lining of the proboscis-cavity. In the 

 anterior third it is evenly distributed over the inner surface, 

 but at the back of the posterior third, where the proboscis 

 tapers abruptly to its stalk, the layer of mesoblastic tissue is 

 much thinner dorsally than laterally and ventrally (compare 

 fig. 13 which is through this region with figs. 13 and 11 which 

 are anterior to it). On passing further backwards, the cavity 

 in Stage Gis divided into two by a great proliferation of 

 mesoblast from the dorsal surface, which grows downwards 

 until it meets the ventral mesoblast. The position occupied 

 by this structure at first coincides with the point at which the 

 anterior mesoblastic pouch closed off from the archenteron. 

 In that stage the anterior body cavity was a simple sac con- 



