270 AYERS ON THE DEVELOPMENT 



hypodermic cells of the pad. In the median dorsal line of the abdomen, in a plane parallel 

 with the dorsal fin muscle, is a band-shaped muscle which serves to flex the abdomen 

 towards the head. The remaining muscles are smaller than those menti < ( ; 1 < a ( an 

 without exception are confined to the abdominal region. In pi. 21, fig. 26, are shown four 

 abdominal muscles which, when viewed from behind, form a rectangular figure about the 

 mesenteron. PI. 25, fig. 10, is a transverse section showing the adductor of the mandible 

 near its posterior margin and its manner of origin and insertion. The striated appearance 

 of the muscle fibres is shown in pi. 24, fig. 28. PI. 25, fig. 12, illustrates the manner in 

 which the abdominal muscles are inserted into the body wall. 



As has been stated, the alimentary tract of the first larva became indistinguishable from 

 the surrounding cell mass as the larva passed into the second stage, but already the stomo- 

 daeum had opened into the mesenteron so that at the beginning of this stage the larva is 

 furnished with as complete an enteric cavity as is present in many insects that are not 

 degraded by parasitism (e. g. carnivorous larvae of Myrmeleon etc.), and there is a resem- 

 blance between such larvae and the second larva of Teleas which is even more than super- 

 ficial. The enteric cavity consists of a blind sac in the course of which there are two 

 enlargements, one terminal, the other (pi. 24, figs. 23 and 25), near the suboesopha- 

 geal ganglion and probably near the point of union of the stomodaeum and mesenteron. If 

 this conjecture is correct, this enlargement is homologous with the proventriculus of other 

 insects. This second enlargement is not always sharply limited. The enteric sac is a cel- 

 lular membrane (pi. 24, figs. 23, 25 ; pi. 25, figs. 1, 7, 8, 17) in which the large, polygonal, 

 thin-walled cells are closely united ; each cell contains a spherical nucleus, central in 

 position, with a varying number of nucleoli, one of which is much larger than the others. 

 The mesenteron or abdominal portion of this enteric cavity is retained in its place by pro- 

 toplasmic filaments which reach out and coalesce with similar processes from the hypodermic 

 and muscle cells, pi. 25, fig. 1. No muscular fibres are to be distinguished in the enteric tract, 

 but contractile motions are sometimes seen, especially in the anterior portion. The oesopha- 

 gus, after passing through the space between the large mandibular muscles and the 

 circum-oesophageal ring of the nervous cord, passes into a depression of the ventral sur- 

 face which is continuous with the furrow in the anterior face of the lower lip. (PI. 25, 

 fig. 19.) Sections through the thoracic and abdominal regions (pi. 25, figs. 10, 11, 13, 14, 

 17) show that the cells of the mesenteron are closely connected with the hypodermal 

 layer and the ventral stripe, but that later the enteric tract becomes entirely 

 separated and is joined to them only by the protoplasmic filaments. The procto- 

 daeum arises as an invagination in intimate connection with the posterior end of the 

 abdominal nerve plate. The hypodermis, which, like the enteric tract, is a single-layered 

 sheet of cells more or less fused together, lies in close contact with the inner surface of the 

 cuticula and in its primitive condition is a syncytium, the protoplasm of which is finely 

 granular and contains nucleolated nuclei. Subsecpuently walls appear about all these 

 nuclei thus forming the polygonal cells of the hypodermis. It is probably while in the syn- 

 cytial condition that the cuticula is secreted, since there is no trace of the outlines of the 

 cells such as would probably appear on the inner surface of the layer if such cell walls 

 existed. The hypodermal cells are o» \bout the same size as the cells of the mesenteron. 

 This layer at its posterior termination opens into the cavity of the caudal appendage so 



