1904.] NATURAL SCIENCES OF PHILADELPHIA. 749 



mic. It has no intestine, and no nervous system, unless, indeed, the 

 large cells of the axis may be considered embryonic nerve cells, or some 

 of the cells on the anterior surface of the diaphragm. It is not com- 

 parable with a head, nor is it to be regarded as a segment of equal 

 morphological value with the head-trunk. The proof is yet wanting, 

 but it will probably be found that this proboscis is the essential larval 

 organ, disappearing more or less completely before the adult condition, 

 and that the head-trunk is the persisting region. 



The posterior head-trunk (fig. 36) is bent upon the proboscis, as the 

 larva lies within the egg-membranes, but by the action of the longi- 

 tudinal muscles the whole body may be straightened. Its hypodermis 

 and cuticula is of the same structure as that of the posterior portion 

 of the proboscis, except for the presence of two pairs of cuticular 

 hooks {H.), placed right and left on each side of the posterior end, 

 and for the presence of a thickening of the hypodermis. This thicken- 

 ing {Nv.) lies ventral just behind the diaphragm, and shows a double 

 row of large nuclei, one row to the right and the other to the left of the 

 median line, and each row with four to five nuclei. This thickening 

 is to be regarded as the nervous system of the larva. It is an ectoder- 

 mal thickening of definite form in every larva; and its ventral position 

 corresponds with the position of the nerve cord of the adult. There 

 are no other parts of the larva which can be considered nervous, unless 

 the large hypodermal cells composing the axis of the proboscis have 

 that function — against which assumption a number of objections 

 might be urged. This thickening is then a double row of neuroblasts, 

 recognizable only on carefully stained preparations; nerve fibres cannot 

 be determined. 



The longitudinal muscle-cells {Mus.) lie just beneath the hypodermis, 

 and terminate anteriorly against the diaphragm. They have the same 

 appearance as those of the proboscis, but are more difficult to perceive, 

 particularly those of the dorsal side. Here also there is no trace of 

 circular muscles. 



In the archicffil, at the anterior end of the head-trunk, is placed a 

 large gland {Gl.), which, as we have seen, arose as an abstriction from 

 the entodermal canal. By a long duct (fig. 36, Gl.d.) it is connected 

 with the exterior at the base of the stilets {St.) of the proboscis. This 

 gland lies free within the archicoel, and contains about eight large 

 nuclei; cell boundaries are bareh^ distinguishable, and the cytoplasm 

 is dense and stains uniformly. The long, convoluted duct can usually 

 be found onh' in life, when it appears as a perfectly clear, convoluted 

 line. On a few preparations, however, its portion within the gland 



