70 T. C. WHITE ON THE HISTOLOGICAL DEVELOPMENT 



nection with this eye, proving conclusively the true nature of this 

 ocellus (fig. 1). In this first segment we may also notice a portion 

 of the pharynx. This is very -wide, and opens as a trumpet-shaped 

 mouth between the two Platygnaths. In its earlier stages it 

 appeared almost destitute of structure, saving for the presence of 

 minute longitudinal stria3, but as development advanced it was seen 

 to consist of two coats, an internal one comparatively devoid of 

 structure, but armed with a number of minute chitinous teeth, and 

 an external coat composed of longitudinal and annular muscles. 

 These two coats are attached loosely to each other by very fine and 

 scarcely visible fibres of connective tissue. This creature has a curious 

 power of everting the whole of the pharynx together with the gizzard 

 and oesophagus when irritated, and this proceeding does not appear 

 abnormal or productive of inconvenience, but rather, if we may 

 judge by the presence of two partially coiled nerves which accom- 

 pany the oesophagus in this eversion, it would seem as if an 

 intentional provision had been made for this performance, the 

 animal swimming about for days with its pharynx projecting from 

 its mouth. 



Passing now from a description of the head, we will examine the 

 second segment, or thorax, and its contents. The most prominent 

 objects which first arrest the attention are two dark reniform bodies, 

 which occupy the centre. These are air-vesicles. They consist of 

 a delicate membrane distended by air, and having their coats 

 strengthened by chitinous rings similar to those met with in the 

 tracheal tubes of the Insecta. The outer surfaces of these sacs are 

 partially covered by patches of some dark brown material possessing 

 one or more nuclei, but of whose nature and purpose I cannot offer 

 any explanation except that later on in this creature's development 

 these patches appear broken up ; at the same time large globules 

 of some highly refractive substance appear in their vicinity. In the 

 earlier stages these air-vesicles appear to have no communication 

 with other parts of the body, nor with each other, nor with two 

 similar air-sacs situated in the ninth abdominal segment, but later 

 on each vesicle undergoes a slight elongation of one of its ends, 

 forming a short tube having a bulb at its extremity, from which 

 ultimately a number of fine tracheal tubes are distributed through- 

 out the adjacent tissues. Near these air-sacs are seen the salivary 

 glands. These are two elongated tubular bags with sacculated walls 

 containing large nuclei. The necks of these bags are prolonged 



