382 SUMMARY OF CURRENT RESEARCHES RELATING TO 



but this change in function does not affect the gills, which became 

 fixed to the thoracic region in the Schizopod stage, which is gone 

 through both phylogenetically and ontogenetically by the Decapoda. 

 In consequence of the raising of the pleura the epimeral walls, and 

 probably also the arthrodial membrane at the base of the appendages, 

 have become stretched, and the gills are no longer situated close 

 together, but are separated. This stretching of the arthrodial membrane 

 and the time at which it took place, the need of protection to the 

 branchiae, the condition of the larva when hatched, and probably also 

 the condition of the tissues of the creature, will probably serve to 

 explain all the various positions in which brancjiise are found. 



In Stomapods the mid-body is developed before the hind-body, and 

 here there are abdominal swimming appendages and branchial tufts 

 attached to the epipodite. The degenerate Cumacea have only one gill 

 remaining. The Arthrostraca are probably to be derived from the 

 Archischizopod, but have from the beginning taken a different line of 

 descent to the true Schizopoda- 



Development of Compound Eye of Alpheus.* — Dr. F. H. Herrick, 

 who has for some time past devoted himself to the embryology of this 

 crustacean, has a short account of the mode of development of the eye. 

 He draws attention to some leading points ; the optic disc is at first a 

 unicellular layer of ectoderm ; this disc becomes thickened by migration 

 of cells from the surface, by delamination, and, probably also, by the 

 addition of cells from the yolk. The optic lobe thus formed becomes 

 differentiated into two parts, which are separated by a structureless 

 membrane. From the outer portion or retinogen, which is at first a 

 single layer, the retina is developed ; the rest of the lobe (gangliogen) 

 gives rise to the ganglia and parts of the eye below the basement mem- 

 brane. The retinulae, retinophorse, and corneal cells are differentiated 

 ectodermal elements belonging to the retinogen. The retinophoree are 

 not prolonged far inwards and do not enter the retinular bundle. There 

 is no swollen pedicle, and nothing answering to pedicle, rhabdom, or 

 spindle has been detected. No invagination or formation of cavities of 

 any kind occur in the development of this eye. The ommatidium 

 consists of thirteen cells disposed in three layers, as follows : (a) corneal 

 layer — 2 cells ; (h) retinophoral layer — 4 cells ; and (c) retinular layer — 

 7 cells. No nerve-fibres were detected in the crystalline cones. 



The author is of opinion that there is a tendency to exaggerate the 

 significance of an invagination of ectoderm, for he thinks it improbable 

 that, at the time when most of these infoldings occur, a cell has any 

 true upside or downside, or that an included cell is differentiated from 

 one next it, which does not share in the invagination. He suggests as a 

 temporary working hypothesis that all invaginations of ectoderm, where- 

 ever they occur in the Animal Kingdom, are primarily of no morpho- 

 logical importance, but simply mechanical expedients for introducing 

 rapidly a large number of ectoderm cells below the surface ; where 

 there is any significance it is secondary. 



Anomura of the ' Challenger. 'f — Prof. J. E. Henderson states that 

 the collection of anomurous Crustacea made by H.M.S. ' Challenger ' is 



* Zool. Atizeig., xii. (1889) pp. 164-9. 

 r t Reports of the Voyage of H.M.S. ' Challenger,' Zoology, xxvii., No. Ixix. f 1888) 

 221 pp. (21 pis.). 



