148 



Mr. W. F. R. Weldon. On a 



[Mar. 3, 



groove is occupied by a single unpaired body cavity (fig. 1, I), which 

 obliterates the blastoccel. This cavity is traversed by a number of 

 longitudinal " mesenchymatous " muscles (Ifc), and carries on its 

 floor a glandular organ (CW.), near which opens, asymmetrically, a 

 short canal, which runs to the middle dorsal line, where it communi- 

 cates with the exterior by a pore (P.). Immediately beneath the gland 

 is a forwardly directed diverticulum of the gut (Oli.). 



It seems impossible to avoid identifying these structures with the 

 proboscis gland and pore of Balanoglossus, and the subjacent " noto- 

 chord"; while the anterior paired body cavity represents the collar 

 cavity, the posterior the trunk cavity, of the normal Balanoglossus 

 larva. A confirmation of this view is given by the appearance, at a 

 slightly later date, of a single pair of rudimentary gills (fig. 2, Br.). 



The ectoderm of the larva, up to the point at which the gills appear, 

 is very thick (figs. 1 and 2), and contains many mucus and other 

 glandular cells; while beneath the eye-spots is a well-developed 

 " Scheitelplatte," and beneath the general ectoderm a well-formed 

 embryonic nervous system, the details of which I reserve for a later 

 paper. 



Just after the development of the gill-slits, there appears to be 

 much variation in the conduct of the larvae obtained ; some exhibit 

 indications of a normal development ; the majority, however, begin 

 from this point to undergo a gradual process of degeneration, accom- 

 panied by considerable increase in size. 



The shape of the most degenerate larva obtained may be gathered 

 from the nearly median longitudinal section (fig. 3) ; where the collar 



Fig. 3. 



