THE PYCNOGONIDS. 55 



check to the results obtained in Phoxichilidium, the less prom- 

 ising eyes of Pallene and Tanystylum were studied. It is not 

 necessary for our purposes to give a full account of the structure 

 of the eyes of these forms, as they showed in all essential points 

 a close agreement with Phoxichilidium. To call attention to a 

 few of the differences may not be out of place, so I have given 

 two longitudinal sections of the eye of Pallene and Tanystylum. 



Fig. 37 is a longitudinal section through the eye of Pallene 

 empusa. The most noticeable difference, when compared with 

 the eye of Phoxichilidium, is in the absence here of any well- 

 marked lens. The chitin is somewhat thicker over the eye than 

 elsewhere, but not sufficiently so to form a definite lens. The 

 three layers of the eye are clearly seen. The corneal hypo- 

 dermis is not so well developed as in Phoxichilidium, and per- 

 haps this is correlated with the non-development of the lens. 

 The middle layer is essentially similar to that of Phoxichilidium, 

 with its nuclei, vacuoles and rods. The bacilli are, however, 

 relatively more abundant and closer together, and each bacillus 

 is relatively longer. The inner pigmented layer is less well 

 developed and forms a narrow yellowish zone around the inner 

 part of the eye. Cross-sections add nothing to our knowledge, 

 and sections at right angles to the raphe show that it, the raphe, 

 is essentially as in Phoxichilidium. 



In Tanystylum we find the same structures seen in the other 

 two as shown in Pig. 38. The chitin over the eye is thicker 

 than elsewhere, and forms a slightly developed lens, being 

 intermediate in structure between Pallene and Phoxichilidium. 

 The corneal hypodermis is much as in Pallene. The outer part 

 of the middle layer shows few nuclei, which indicates it is near 

 the median plane of the eye. The bacilli are fewer than either 

 those of Pallene or Phoxichilidium. Fig. 39 is a part of a section 

 across the raphe, and, passing through it, shows the arrange- 

 ment of the cells on the two sides. It was impossible to trace in 

 these eyes the nerve fibers after they disappear in the pigment 

 layer. They seem, however, to run around the sides of the eye, 

 probably innervating the pigment cells in their course, and 

 enter the middle layer from the sides and at its outer part 

 beneath the corneal hypodermis. 



