n6 SCIENCE PROGRESS. 



Leydigii, which is a true cave animal, branched cells are 

 present in the subcuticular syncytium, which resemble the 

 chromatophores of allied types in all points except that 

 pigment is not deposited within them. 



In Idotea, according to Matzdorff (2), the chromatophores 

 lie exclusively in the epidermis. This consists of two 

 layers in Idotea, an outer epithelium of chitinogenous cells 

 and an inner granular syncytium with regular distributed 

 nuclei but without cell-boundaries. The chromatophores 

 lie in this under layer, and occupy its whole thickness, even 

 bulging slightly into the epithelial layer above and into 

 the connective tissue below. Each chromatophore is a 

 naked nucleated cell without a bounding membrane. Its 

 shape can vary from a spherical or oval to a highly stellate 

 form. The processes bore their way through the proto- 

 plasm of the syncytial layer without having any definite 

 paths prescribed for them ; and the branches of neighbour- 

 ing chromatophores can fuse to form an intricate net-work 

 of pigmented tissue. The chromatophores of Isopods are 

 true amoeboid cells whose resting condition is that of con- 

 traction, and whose active condition is that of expan- 

 sion. 



In Caprellids, according to Paul Mayer (3), the epi- 

 dermis is cellular, and forms a true pavement epithelium 

 which is one-layered throughout — a statement which he 

 would extend to Arthropoda in general. He agrees with 

 Hoek (4) that the chromatophores are exclusively situated 

 in the connective tissue, and in the envelopes arising from 

 it which surround the gut, nervous system, gonads, and 

 heart. 



We thus see that, if the observations of Max Weber 

 and Matzdorff be well founded, the Crustacean chromato- 

 phore, though frequently assuming a mesodermal position, 

 is very probably of ectodermal origin. Mayer's generalisa- 

 tion that the epidermis of Arthropods is universally one- 

 layered cannot be absolutely accepted, for it has been shown 

 by Viaillanes (5), Minchin (6), and Mingazzini (7) that in 

 insects at any rate certain branched cells of the epidermis 

 may be found beneath the general level of the epithelium. 



